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- "Fifth Floor Guardian Cocytus, at your command."
- — Cocytus' Introduction
Having an enormous body size of 2.5 meters, Cocytus has the appearance of an insect walking on two feet - a fusion between a mantis and an ant. With a tail twice as long as his height, Cocytus is covered in sharp spikes like icicles and has a strong jaw that can easily snap people's hands.
Two of his hands hold a silver halberd, while the remaining two hands hold a mace emitting black light and a crooked shape sheath which seems to be for a broadsword. With a breathtaking cold air, the pale blue, hardened bone armor oozes out diamond dust like bright light. His shoulders and back look like uplifted icebergs.
Cocytus has a warrior personality, respecting anyone with a fighting spirit, and the heart of a warrior. As a Floor Guardian, he is in charge of Nazarick's martial defense. Cocytus is loyal to a fault that he even allows his master to fight the rebel Shalltear alone as the former was ordered to do. This is shown when he prevented Demiurge, under Ainz's order, from interfering in Ainz's fight against Shalltear, even though he too also feared the possibility of losing his master. As Cocytus is a warrior by heart, he likes a good battle and has a sense of honor. He is not above the thought of condescending his opponents, but rather show admiration of their tenacity to fight. Unlike his isolationist and xenophobic colleagues, he is more accepting and open to the idea of allowing outsiders like the lizardmen to join Nazarick. He also appears to enjoy watching how battles play out as a bystander, excited to see what tactics and strategies his enemy use.
Though Cocytus takes pride in his position as Nazarick's defender, he is somewhat envious of his fellow Floor Guardians. Cocytus likens himself as the 'sword' of Ainz Ooal Gown, that executes his master's wishes. He thinks the only way he could prove his worth to Ainz is by actually defeating the invading forces that would pose a threat to Nazarick. Cocytus is not fond of asking for assistance in order to save face, but when the need arises, he will put his pride aside to accomplish his task. For that reason, he was willing to accept a personal defeat but not one that will disgrace and humiliate the Great Tomb of Nazarick and the 41 Supreme Beings. After losing his army to the lizardmen, he starts to reflect more on his actions and no longer wishing to obey his master's order blindly without first offering his own input on it. With the task of now administering the Lizardman Village, he has also began to shoulder more responsibilities that were expected him to carry out by his master.
Knowing his defeat, Cocytus doesn't mind if his master reprimands him. He was tolerant about it to the point that he had already prepared himself to be slain if given the order to do so. In other words, Cocytus was ready to commit suicide in order to erase the stain of his dishonor for losing to the enemy. What Cocytus felt most afraid of is the notion that he could possibly disappoint his master and be abandoned because of it. He was sensitive of his failure having an affect on the rest of his fellow NPCs that his death alone will not be enough to atone. Cocytus failing his first campaign put a mental toll on his state of mind where he tried his best to console himself from seeing such a scenario happen. When seeing Ainz go out of his way to do tasks still, Cocytus can't help but sometimes also feel sadly inadequate and incompetent as he himself couldn't be able to fulfill the work in his master's stead. He is also a person who is diligent with whatever task was given to him such as training either alone, with Ainz or with the lizardmen in swordsmanship.
Ever since the end of the Lizardmen War, he has become like a teacher-like figure to the lizardmen, strictly teaching them to abide by the rules of Nazarick and how they respectfully act around Ainz. This causes Cocytus to become a more outspoken person about the decisions he needs to wisely and carefully choose as Ainz wishes him to think of ways in bettering Nazarick. As a result, he starts to develop confidence and trust in his judgement of what to do rationally during wars. Cocytus yearns for Ainz to produce an heir for Nazarick, to which he could look after and train under him. He was willing to train either Ainz or his future child about direct combat physically and how to approach these kind of foes. Based on his conversation with Shalltear, he fondly enjoys the idea of honing and crossing blades with his master in the ways of swordsmanship if permitted the chance to duel him. He takes thrill in dueling worthy warriors who caught his attention, looking to challenge their resolve head on and even possibly, recruit them if a chance presents itself.
Cocytus was created by Warrior Takemikazuchi as the Floor Guardian of the 5th Floor of the Great Tomb of Nazarick. In his setting, he was programmed to possess every sort of knowledge on all of the 41 Supreme Beings' Divine Weapons. Nonetheless, Cocytus admits that he had never once seen such weapons and its powers utilized in battle before until his master Ainz Ooal Gown equipped it in combat against Shalltear Bloodfallen. Like his creator, he also sees himself as a prideful warrior who admires the fighting spirit of his foes, giving them the honor and respect they deserve when faced against the former in combat.
Whenever he emerge triumphantly against his adversary in combat, Cocytus tends to lay claim of ownership over the weapon used by them, adding it to his own collection of weapons regardless of its value. Furthermore, Cocytus would even considered keeping the corpse of the warrior he slain as trophies for safekeeping, making sure to have them kept well-preserve and intact via ice to bring back home with him. No matter how strong or weak the warrior is to him, Cocytus approach them without showing any bias or arrogance all so that the former seeks to have a proper match where they can fight to the death.
The Undead King Arc
- Main article: The Undead King Arc
When Momonga calls for the Floor Guardians to the 6th Floor, Cocytus is the second to arrive after Shalltear. During his arrival at the Amphitheater, he stops the argument between Aura and Shalltear in front of their master. As there were no intruders on the 5th Floor of Nazarick, Cocytus briefly discusses with Ainz about his free time there where he has been training in order to be ready for deployment at any time. Hearing Ainz's praise of his hard-work, Cocytus appreciates his master's kind words and feels that the task was no longer tiring for him to do. The brief discussion between the two ends when Cocytus informs his master that the last two of the Floor Guardians, Albedo and Demiurge, have arrived.
Once all of the Floor Guardians have arrived to their meeting place, Cocytus pays his respects with the rest of them to Momonga, describing the greatness of their master. Right after Momonga leaves, Cocytus and his fellow Floor Guardians share what their impression is towards Ainz after feeling the power of his Despair Aura emanated by his master. Along the way, Cocytus conducts talks with Demiurge and Mare about the future of Nazarick.
At first, he thinks it is disrespectful of them to consider Momonga's successor when they are suppose to serve their master faithfully as they were created for this purpose. However, Cocytus's thoughts change the moment Demiurge reasons with him that they should also consider pledging their loyalty to Momonga's future heir as well, much to his interest. He begins daydreaming of a possible scenario where Momonga has an heir and that it starts calling him its Uncle. He happily imagines himself as a cool old uncle, loyally serving Momonga’s heir, until Demiurge brings him back to reality. According to Cocytus, it is everything he has ever dreamed of happening.
After he and the rest of the Floor Guardians are dismissed by the Overseer to carry out their respective duties, it is said that he was tasked with supervising Nazarick's perimeter security.
The Emissary of the King Arc
- Main article: The Emissary of the King Arc
Cocytus is paid a visit by Ainz's Death Knight on his floor. The Floor Guardian shows his guest hospitality, even shaking the undead's hand as they serve the same master. After the undead asks questions on who Cocytus is the friendliest with in Nazarick, he expresses that it would be Demiurge, Kyouhukou, Grant, and Gashokukochuuou. He states that there is no creation of the 41 Supreme Beings whom he do not get along, having only speak to them but not met them in person. For that reason, he wanted Ainz's permission to arrange some time in visiting them and deepen their friendship. As for his other desires, it would be to spar with Sebas Tian, Albedo, and even Ainz if permitted. Another wish is that as a loyal warrior, Cocytus wants Ainz to have an heir who he can look after and protect so that the future of the Guardians will be secure. At the same time, he wants the chance to train said heir on how to deal with any enemies such as melee attackers that may try to harm it while under his protection. Without one, he fears that Nazarick denizens will result to infighting disputes within the tomb emotionally and ideologically.
The Guardians' Vacation Arc
- Main article: The Guardians' Vacation Arc
When Ainz suggests giving the Guardians vacation off from their work time, Cocytus tells his master that he is doing fine and not overexerting himself, much to his master's insistence. Cocytus informs Ainz that he cannot grow any larger than his current form, telling him that he has already completed his life cycle. He recommends his master to let him use items as a substitute for sleep. In the end, he gives in to Ainz's order and decides to take a break and rest.
During a private meeting with the Guardians, he allows Sebas entry into their conference room under the pass code: All Glory in this World Belongs to Nazarick. Cocytus acknowledges that Demiurge's prediction was spot on about Ainz continuing to work while they, on the other hand, are resting. Seeing the results of Ainz's earlier health examination by Sebas, he thinks it is not a good sign for their master. He believes the color of the health exam represents some sort of meaning like "So black-hearted I'm going to die" that are unclear to him much to his confusion. Cocytus presumes that it means Ainz is under the influence of an unknown World Item's effect.
Cocytus reassures everyone that what they do for Ainz should not be taken as disloyalty as long as they are having this discussion for the sake of their master. He admits to not being able to pick up on Ainz's hidden meaning of why their master decided to give them break time, respecting Demiurge's keen insight on how that may have been the case for them.
The Bloody Valkyrie Arc
- Main article: The Bloody Valkyrie Arc
Cocytus is ordered to restrain Demiurge from interfering with the fight between Ainz and Shalltear. As a result, when Demiuge attempts to go and assist his master, Cocytus quickly blocks the Floor Guardian from leaving by brandishing his weapon in front of him
While doing so, he engages in conversations with Albedo and Demiurge about Ainz's decision to fight the vampire alone. Cocytus fins his master's strategy when dealing with Shalltear to be strange, much to his confusion. He reasons with Albedo that a sequential attack would be safer to do where they can attack in waves and wear down Shalltear’s HP and MP, which Ainz would have done rationally. His reasoning is met with acceptance by Demiurge as he and the demon both agree that their master was lying to them in order to hide something.
Nonetheless, Cocytus and Demiurge are told by Albedo that Ainz is a different person than he was before Shalltear's betrayal, which is why the two are advised to have faith in their master. To inform the two about Ainz's chance of defeating Shalltear, he states that it was seven to three.
When Demiurge threatens Albedo to step down as the Guardian Overseer in the event that Ainz loses the battle with Shalltear, he is angered by his colleague's words. He calls Demiurge's suggestion as treacherous since that position was given to Albedo by the 41 Supreme Beings.
As he watches how the fight between Ainz and Shalltear plays out, Cocytus can't help but voice his astonishment, amazement, and excitement at seeing his master freely use the equipment and items formerly used by the Supreme Beings respectively. Although Cocytus is more confident than ever that Ainz can win the battle against Shalltear, Demiurge thinks otherwise due to his lack of warrior knowledge. Cocytus enlightens him that Shalltear abandoning her defense for an all-out attack is what he would have also done if he was in the same situation as her. Although Demiurge is impressed by his explanation, Cocytus doesn't think so highly of his knowledge as he only knows the weapons' effects and names.
With the battle almost reaching its conclusion, Cocytus acknowledges and admires how the entire fight from start to end has gone all according to Ainz's plan. After Ainz emerges victorious from his battle, he and the other Floor Guardians beg him to never do such a thing again, and to allow them to handle any future incidents that may come from here on out. Once Shalltear is revived, he advises Ainz to teach the vampire a stern lesson for messing up on not only her mission, but for betraying their master and Nazarick as well.
The Show Must Go On!
- Main article: The Show Must Go On! Arc
Cocytus is among the judges of the auditions in selecting the person who will play Juliet. He is in disbelief that Demiurge supports the idea for a competition between the two Floor Guardians, Albedo and Shalltear. Later on, he is seen training on the 5th Floor and is approached by Shalltear. Cocytus, knowing something suspicious is up, attempts to rebuff her. His suspicion is proven when she tries to bribe him to vote for her in the auditions. However, when she offers the role of Tybalt to him, for a chance to exchange blades with Ainz, he is tempted by the idea. Cocytus, however, being an honorable warrior, refuses and tells her to leave. During the audition, Cocytus is moved by Albedo's performance, which results in him voting for her.
The Sealed Evil Tree Arc
- Main article: The Sealed Evil Tree Arc
Cocytus is summoned to the center of the Tob Forest to fight against Zy'tl Q'ae. He is ordered to fight as a team with the Floor Guardians. However, he keeps forgetting and attacks the treant twice with overwhelming force until he is reprimanded by Albedo to reevaluate their master's order.
The Lizard Man Heroes Arc
- Main article: The Lizard Man Heroes Arc
Cocytus is ordered by Ainz to annihilate the lizardmen and command the undead army of Nazarick. During the war council, he instructs his vassals to tidy up their table so that Entoma can prepare a spot for the set of Message Scrolls and Teleportation Scrolls to be placed on. Once Cocytus receives the scrolls, he is informed by the insect maid that they are made with the skin that Demiurge works hard to obtain. He is told to contact Ainz for feedback if any problems arises regarding their use. Nonetheless, he is bitter to see that Demiurge has pulled even further ahead of him after aiding his master with the making of these scrolls.
At the same time, Cocytus hopes to prove his devotion and loyalty to Ainz by obtaining good results where he wins the Lizardmen War under certain conditions and rules placed as restrictions on him. In other words, his task is to achieve victory using only the forces deployed to the lakeside. Cocytus begins his task, casting one of the Message Scrolls and giving his orders to Iguva=41, the commander of the undead army. He watches how the destruction of his undead forces playes out from behind the scene as the Lizardmen Alliance devastates it in warfare. Cocytus’ opinion of the Lizardmen goes up by a notch, and he concludes that they are not a foe that can be instantly destroyed. That being the case, Cocytus figures the only way he can win the war against them is to draw out the battle. Cocytus listens to the suggestions offer by his vassals, borrowing their wisdom in order to dictate what his next course of action will be before then casting another Message Scroll once more to the undead commander.
Witnessing how his army is almost close to being defeated by the lizardmen, Cocytus uses a Message Scroll to call on Demiurge and request his advice. Cocytus is offered several hints and cues by Demiurge on why Ainz gave him such low-tier undead forces to begin with. From his conversation with Demiurge, Cocytus learns that he must divine his master’s intent and adapt his methods to the situation, not follow his master's orders blindly. It is for the first time that Cocytus has started to contemplate his master’s true underlying motives. Cocytus's brief discussion with Demiurge about Ainz's true objectives for him comes to an end when Entoma stops them from discussing any further.
He deploys his trump card, the undead commander Iguva=41, onto the battlefield to engage the lizardmen in combat. Seeing Iguva=41 being destroyed and his army being defeated by the lizardmen, he realizes that his mission to annihilate the lizardmen has failed. Although he is deeply impressed by the lizardmen's victory during the war, Cocytus is surprised at his defeat and feels ashamed for losing. This leaves him no other choice but to return to Nazarick with a sense of defeat. While walking forward to the Throne Room, he thinks to himself what his fate may be and how the last Supreme Being remaining will forsake him, making the former feel ashamed in fear for what is to come next. Cocytus apologizes for his late arrival to the gathering place and is then introduced to Victim, a fellow Floor Guardian he did not know of until now. Once Ainz and Albedo arrive, he and the rest of the Floor Guardians are guided by Demiurge to the Throne Room where the two await them.
During the meeting, he prepares to apologize, but is stopped by Ainz and Albedo. He is ordered to explain what his feeling are as a commander of a defeated army. As a result, Cocytus describes to his master that he is feeling deeply remorseful for his own inability to achieve victory over the lizardmen, along with losing Ainz's personal undead creation. Unable to give the proper answer his master wants, Cocytus is asked once more by Ainz how he felt when commanding the battle and what he learned from it. Hearing this, Cocytus understands what Ainz expected from him, explaining the various ways to his master what he should have done beforehand to defeat the lizardmen. However, Cocytus acknowledges that he did not do any of that mentioned by him as he assumed his forces alone were enough to overwhelm them.
After carefully analyzing his failure and hearing that his master wants him to wipe out the lizardmen as punishment, he hesitates on following through with that order, refusing the thought of doing so. Hence, Cocytus asks Ainz if they could show mercy to the lizardmen and take them in under Nazarick's rule. He reasons that they can be used to breed future warriors powerful enough to serve Nazarick while instilling to them a strong sense of loyalty. However, since this is not sufficient reasoning to convince his master, Cocytus is assisted by Demiurge, who reasons with Ainz that they can use the lizardman to experiment on Nazarick's rulership. With Demiurge's idea now put forward, Ainz accepts it and allows Cocytus to spare the lizardmen.
In order to redeem his failure and as punishment, Cocytus is given the order by Ainz to rule over the lizardmen without fear and terror while instilling a deeply-rooted loyalty to Nazarick within them. Cocytus even receives permission from his master to call on the aid of others, which will require considerable resources, manpower, and rations. Once the meeting is over and his master leaves, the first thing Cocytus does is express his gratitude to Demiurge, admitting that the lizardmen would have been exterminated if not for the demon's intervention. Cocytus is informed by Demiurge that the reason their master approved of his suggestion in the first place is because Ainz himself had foreseen and anticipated such a development happening.
Later on, he is ordered to first do battle alone against the lizardman heroes. Along the way, he cleaves a path through the forest to find the lizardmen, cutting down several gigantic trees that fall to the ground with earthshaking tremors at its wake. Since his master is watching over him and his battle, he ensures the lizardmen that they will get a chance to shine. Before the battle happens though, he creates two pillars of ice ahead of time, warning any lizardman who cross it must be prepared to forfeit their lives when fighting him. Seeing that only Shasuryu Shasha, Zaryusu Shasha, Zenberu Gugu, Kyuku Zuzu, Sukyu Juju, and fifty seven lizardmen have the resolve to fight, Cocytus then gestures these group of lizardmen to begin doing battle with him. Cocytus decides to cull their numbers first by vanquishing the fifty seven lizardmen with his [Frost Aura] skill. When this is done, he starts dealing with the five strongest lizardmen and the two Swamp Elementals simultaneously, taking care to slay them one by one until he emerges as the victor of the bout.
After that quick battle, where Cocytus kills all of them with his hands, he returns to Ainz's side and is informed by his master that he cannot rule over the lizardmen through fear. Because Cocytus emerged victorious from his battle against the lizardman, he is now in charge of administering the lizardman tribes and fostering deeply-rooted loyalty to Nazarick. Cocytus is expected by his master to cultivate the growth of talented members of their species. When his master plans to turn the corpses of the lizardmen slain in battle into raw materials, he successfully persuades Ainz to resurrect the strongest lizardmen back to life rather than go through with that idea. He reasons that not only do these lizardmen have the warrior spirit to grow even stronger, but it will also allow his master the chance to conduct practical experiments with resurrection magic on them. Cocytus later brings in Crusch Lulu as the representative of the lizardmen to handle talks of negotiations with his master regarding the fate of their species.
The Two Leaders Arc
- Main article: The Two Leaders Arc
At some point after his war with the lizardmen, Cocytus has a disagreement with the species over their suggestion of returning to their former village to fish. Knowing their reason for that is due to food shortages from not having enough preys to hunt or trap, Cocytus, with help from Demiurge, proposes the idea of importing rations from Nazarick and distributing it to them using Cauldron of the Daghdha. Later on, he begins studying methods to ensure a sustainable food supply. The solution he discovers is the fish farms built by Zaryusu. After that, he discusses the matter with Demiurge and begins to build superior fish farms.
Cocytus leaves Snowball Earth, 5th Floor to his servants, the Frost Virgins, to defend in his stead while he departs from Nazarick to the Lizardman Village. Once there, it is decided that Cocytus and the lizardmen will to go on a mission together. According to Shasuryu, Cocytus brings several of his subordinates and several dozen trainee lizardmen on his expedition to subjugate the Toadmen in the northern part of the Great Lake. After the conquest, he returns to the Lizardman Village and receives a message from Mare that Ainz has invited him and the male Floor Guardians to take a bath with him. Cocytus accepts the invitation and later heads over to Nazarick and meet up with his master, Demiurge, and Mare.
While proceeding to the Spa Resort Nazarick baths with his master on the 9th Floor, he notices that there are fewer Eight Edge Assassins around in the tomb. He is informed by Ainz that they're at the E-Rantel inn with Narberal Gamma. Hearing this, he shows his concern for the battle maid, knowing that she is alone by herself and treated as bait for any enemy to come and attack. Finding it hard to keep his cool emotionally, he asks his master several questions about whether the enemies will take the bait in a way that enables them to mobilize their forces afterward. He learns that Ainz has every intention of making the one responsible for Shalltear suffer as much as the latter did to her.
In addition, Cocytus informs Ainz of his plan to temporarily return to his position of guarding Nazarick since the matters which require his personal presence at the lakeside have all been concluded. Learning that Ainz still has some tasks to do, he finds it hard to accept how his master is persistently handling work while NPCs like them are supposed to do it for him. To keep his master at ease, he reassures Ainz that he is the ruler of Nazarick and it is his duty to obey every word he says. At the same time, he feels gloomy, knowing his inability and incapability to assist Ainz on his tasks like Demiurge can. Cocytus is consoled by Ainz when the latter comforts him, saying that he will one day be on par with Demiurge as long as he continues learning by doing other things like ruling the Lizardman Village.
While waiting for Demiurge and Mare to arrive in the Spa Resort Nazarick with them, Cocytus and his master are met by the female Floor Guardians, including Albedo in particular, who attempt to assault him once more. Cocytus cuts his way between Ainz and Albedo, protecting him from her attempt to assault their master. He is shortly later joined by Demiurge and Mare, who arrive to see their master being troubled by Albedo's action, readying his halberd to do battle with the Overseer if she continues to act that way. Nonetheless, before things escalate even further, the Floor Guardians are reprimanded by Ainz to stop this nonsense and head to their respective men and women's baths.
While bathing, he expresses his fondness for taking a cold bath while others convey their views on what they like. Cocytus' exoskeleton body is discussed by the Floor Guardians, much to the latter's pride and annoyance of thinking he is naked all this time. Due to the interruption of the female Floor Guardians, who are attacked by the Lion Golem, he and his fellow male Floor Guardians are ordered to assist them in stopping the entity. Although Cocytus is initially confused on whether it is a Area Guardian and why it is in the women's bath, he learns from Ainz that that golem's voice is Luci★Fer.
The Dark Hero's Story Arc
- Main article: The Dark Hero's Story Arc
During the Floor Guardians' meeting on figuring out what to ask Ainz as compensation, clothing is brought up. Cocytus says clothes are unnecessary for him, earning him a tease from Aura that he is naked. He later suggests asking Ainz for weapons as he is interested in adding to his collection and also to provide new equipment for the lizardmen under his charge. When Aura wants to have Rororo, Cocytus furiously denies it, stating he will not allow his charges' property to be taken and will even bring the issue before Ainz's attention should Aura escalate it. He, however, considers Aura's request for keeping the hydra's children.
Ainz Raises Money Arc
- Main article: Ainz Raises Money Arc
Cocytus is present with his fellow Floor Guardians at a meeting with their master Ainz Ooal Gown. During the meeting, it’s revealed that he has requested a voucher to fight with Ainz as his reward. In addition, he formally requests as the Overseer of the lizardman to prevent the seizure of Rororo by Aura, which Ainz deems reasonable.
Like the others, he is given three gold coins to be used as his salary by Ainz. In the next meeting, he tells his liege how he made the lizardman convert the gold coins into tribal jewelry.
Ainz, disappointed that his subordinates refuse to part with the money, gives them orders them to spend. Finding little interest in the goods offered in E-Rantel, the Floor Guardians seek advice from Sebas Tian and Narberal Gamma. Noticing the fan goods of Blue Rose, it prompts the Guardians to use the gold to create their own brand of fan goods based on Momon the Dark Hero.
The Men in the Kingdom Arc
Ainz is tasked with preparing the funds for rebuilding and financial aid for the Lizardman Village, in accordance to Cocytus’ wishes.
Cocytus is present with Ainz as a witness when they are questioning Sebas about his possible betrayal. Cocytus keeps his distance from Sebas, maintaining the ideal range for him as the warrior to launch an attack. He and Demiurge direct their murderous intent at Sebas and Tuare until they are reprimanded by Ainz to stop while their master continues his interrogation on the two. No longer by his master's side, Cocytus shifts his position to stand behind Tuare, awaiting orders. When Ainz orders Sebas to kill Tuare, Cocytus is ordered to interdict the butler's strike beforehand, judging that the attack was intended to be instantly fatal. After verifying Sebas' loyalty, Cocytus is asked for his opinion concerning his decision to allow Tuare to join Nazarick, which he has no objections to.
When Sebas is given alone time with him and Demiurge, Cocytus tells the former that he does not mind the butler addressing him in a formal manner. Seeing Demiurge and Sebas squabble over the fate of Tuare, he tells the two to mindfully keep their act and composure together in the face of their master before them.
The Invaders of the Large Tomb Arc
- Main article: The Invaders of the Large Tomb Arc
Based on the Pleiades' brief discussion about the intruders, Cocytus appears to be leading the command and overseeing the battle with them. The Pleiades, while watching from afar, witness the downfall of the invaders and note how everything is easily going according to Cocytus' prediction.
The Magic Caster of Destroy Arc
- Main article: The Magic Caster of Destroy Arc
Cocytus is present in the Throne Room when Ainz Ooal Gown welcomes the imperial delegation from the Empire. Later, he and the rest of his fellow Floor Guardians hold a meeting with Ainz, where he discusses his impressions of the Baharuth Empire and its Emperor. Like Shalltear, Cocytus thinks Nazarick doesn't necessarily require a just cause to conquer the Empire until Demiurge reasonably explains it to him in details about the extent of their master's plan. Hearing that the Empire is plotting to have them betray each other, Cocytus believes the Emperor does not know what the meaning of loyalty is. When deciding on a ruler-type name suitable for their master, he suggests addressing Ainz Ooal Gown as the Sorcerer King for their newly founded nation.
The Ruler of Conspiracy Arc
- Main article: The Ruler of Conspiracy Arc
The Craftsman of Dwarf Arc
- Main article: The Craftsman of Dwarf Arc
According to Ainz, Cocytus is not only in charge of the Great Lake, but is now in charge of the Great Forest of Tob, which also makes him a project leader of sorts. Entoma informs Ainz that Cocytus wishes to send over a Lizardman as emissary to him with a report concerning the village’s status and seeks his permission to open a Gate, which their master permits. He is told by the lizardman Kyuku Zuzu that Cocytus wishes to present something to his master.
Cocytus greets and pays his deep respect to Ainz when he comes to the Great Lake to talk with Zenberu on the Dwarf Kingdom. When Ainz offers a reward to him, Cocytus states that there is no need for one as loyalty to his master is sufficient enough for the Floor Guardian. Not what his master wants to hear, Cocyus is given a new order to tell him what he wants within a week’s time, limited to material objects, which he obliges to do, much to his distress.
Later, Cocytus presents the lizardman Zenberu to Ainz for questioning as commanded by his master. He appears clearly outraged when Zenberu looks to discern Ainz's intention, seeing that the lizardman is hesitant to answer his master's questions about the Dwarf Kingdom. Nevertheless, Cocytus's frustration with the lizardman is held back at the order of his liege. Under Ainz's order, from this moment forward, Cocytus is forbidden to harm Zenberu's friends in the village even if the lizardman chooses to oppose his master.
After the misunderstandings are cleared up, Ainz informs him of his intentions to go to the Dwarf Kingdom and forge diplomatic relations. Also, Cocytus is willing to offer himself up to his master so Ainz can sit on his back as a seat, mimicking Shalltear's situation as punishment for having the lizardman act disrespectful towards him. Cocytus asks his master if what he did pleases him or not.
Hearing his response, Cocytus goes on to state whether his master prefer either him or Shalltear as a fitting chair for Ainz, to which the latter responds that the vampire has done it slightly better than he did. However, Cocytus is not without acknowledgement from his master as Ainz considers his sitting position to have advantages to it, noting the cold, cool sensation he emanate would be best on summer days. He informs Ainz that he plans to practice this sitting position for when he must someday bear his master upon his back. In a way, his action impress the lizardmen of Ainz's power over him.
When Ainz asks for his opinion on whether children of different races can get along well with each other peacefully, Cocytus believes that what he suggested should be possible under his will.
Before setting out, Cocytus is wary of the potential dangers and wants to personally escort the Sorcerer King during his journey to the Dwarf Kingdom, suggesting to his master that he is willing to mobilize his forces at any time. However, Ainz convinces him that should he be in peril or negotiations deteriorate, Cocytus will be called upon to his side on the battlefield if the situation arise.
Nazarick Mythology Arc
- Main article: Nazarick Mythology Arc
Cocytus attends a meeting to create a mythology of his master in order to solidify his hold over the masses. In his myth story, he uses the tone of a classic Japanese children folk tale. Taking place in a small town, a boy witnesses the destruction of all he holds dear at the hands of monsters. The boy defeats the monsters using a mysterious power, becoming a hero to the people. A sorcerer named Ainz Ooal Gown appears before the boy, claiming to be his father and has been using him to lead the foolish humans in order to stabilize the peace. Seeing his work complete, he disappears, but promises to return to guide the humans once again. The Guardians then comment that Cocytus' story will be popular among children and adventurers.
The Paladin of the Holy Kingdom Arc
- Main article: The Paladin of the Holy Kingdom Arc
One of the Greater Doppelgängers in Erich String Orchestra tells Ainz about Cocytus requesting that they record their battle with him. This way, Cocytus believes that everyone in the Great Tomb of Nazarick can learn from the battle, which his master permits the group to do.
While fighting Jaldabaoth and Erich String Orchestra disguised as his demon maids, Ainz recalls the practice he had with Cocytus, the method of evading his opponent by closing in with them several hundred times. Ainz notes how one in every ten times, if Cocytus was not attacking in earnest, he could completely slip past them. He remembers Cocytus saying that a good warrior would never make such incredibly flashy and telegraphed attacks.
The Witch of the Falling Kingdom Arc
- Main article: The Witch of the Falling Kingdom Arc
In the Audience Room, with permission from the Floor Guardian Overseer, Cocytus is present when Shalltear brings in Hilma Cygnaeus to present Ainz a witness who can explain their questions about the attack on the Sorcerer Kingdom's trade caravan of grains. Every Floor Guardian, including Cocytus, during the meeting approves of the punishment Ainz has suggested for Hilma Cygnaeus to do. When that matter is concluded, discussion about Philip resumes where it left off while the Overseer explains their strategy for the Fall of the Re-Estize Kingdom.
With Ainz permitting questions to be asked by the Floor Guardians, Cocytus is the second person after Aura to question the strategy of their operation. Cocytus wants to know why they didn't make use of the other noblemen besides Philip during the beginning of the operation. Demiurge answers Cocytus' question, stating that they did recruit someone like that already by blackmailing Philip's father, Baron Montserrat. It wasn't just that nobleman alone, but Demiurge also supports Cocytus' way of thinking, choosing to spare an expendable portion of the nobility who are at least somewhat capable and worthy for Ainz to rule over.
When Demiurge asks if the punishment should be on Philip alone, Cocytus refuses to have it just end there and suggests the same punishment to also be imposed on that nobleman's master.
A month later, after the Sorcerer Kingdom's declaration of war against the Re-Estize Kingdom, Cocytus contributes to the campaign by destroying a city. He also employs a tactic to successfully conquer several cities without problems using an undead army borrowed from his master. His work is commended by Ainz during a meeting with the Floor Guardians in Nazarick.
Upon seeing the death of the Death Knight and the Death Warrior by the flying armor giant, Cocytus is among those besides Demiurge and Shalltear to discover the reason why Ainz did not sent so many forces into the city of E-Naüru. This causes Demiurge to praise him and Shalltear for understanding the reason behind why their master made the decision he did by sending in weak forces.
Once Ainz arrives at the Re-Estize capital with his 10,000 undead army, Cocytus happens to be present by his master's side alongside the other Floor Guardians. When Albedo informs Ainz that they should decline the envoy Zanac's request, he disagrees with her suggestion, reasoning that it would reflect badly on the people's impression of Ainz by choosing not to confront the human. It is also revealed by Ainz that Cocytus is the one in charge of laying siege to the capital. Since Cocytus will be involved, Ainz has given the Floor Guardian the order to watch out for the Razor Edge sword later that is currently in the hands of Brain Unglaus.
After Ainz returns from his meeting with Zanac, Cocytus has a discussion with both Aura and Mare about the upcoming siege of the capital. With Cocytus as the leading commander alongside Mare, who is second-in-command after him, he and the Floor Guardian are permitted to use the full extent of their abilities. Not only that, but he and Mare are also told by their master to not let any citizen of the Re-Estize Kingdom go unharmed once the battle starts. Once the battle begins, Cocytus and his Frost Virgins set off from the front he has established in front of the capital. Cocytus is in charge of laying the capital under siege and upon raiding it, he is later confronted by the warrior Brain. Accompanied by his Frost Virgins, he introduces himself to Brain while the latter does the same.
Respecting Brain's resolve and warrior spirit, Cocytus stikes his Decapitation Fang into the ground and takes out his God Slaying Emperor Blade before assuming a battle stance against him. Cocytus closes the distance between himself and Brain, who is now within his range, and easily ends the warrior's life with one single slash from his weapon. Although the two attack each other at the same time, Cocytus emerges unscathed from the blow, as Brain's blade was not able to reach him. After swinging the God Slaying Emperor Blade, Cocytus shakes off the blood and fat from the katana and returns it back to his Item Box. With the one-on-one fight over, Cocytus pulls his halberd out from the ground and examines Brain's body.
Once he takes Brain's katana weapon and commands his Frost Virgins to freeze the warrior's corpse, Cocytus resumes his walk to Ro Lente Castle. After reaching and getting inside Ro Lente Castle, he proceeds to stride towards the throne room. Cocytus enters the room and stands by his master's side as he sits upon the throne. While this is happening, Cocytus orders his Frost Virgins to stand watch outside the throne room and be on the lookout for Climb, instructing them to not harm the human and allow him entry. Due to the success of the operation ending with the Fall of the Re-Estize Kingdom, Cocytus is also one of the Floor Guardians to have fulfilled his role in the mission without any problem.
- The name "Cocytus" is a reference to Dante's Inferno, in which Cocytus is the ninth and final layer of Hell. It is referred to as a frozen lake that houses all of humanity's traitors.
- In the Web Novel, Cocytus is put in charge of the security from the 9th Floor to the 10th Floor. Cocytus commands a troop of armored beetle warriors. Additionally, he is the creation of Blue Planet.
- Cocytus has four mercenary-type NPCs that serve as his vessels, which consist of three insectoids: a mantis, ant, and another fighter-type insect, and a giant brain.
- If not for the battle with the lizardmen, it was believed by Cocytus that he would have, without question, accepted the order of wiping away his shame and that of Nazarick with slaughter.
- (To Shalltear and Aura): "You. Stand. Before. A. Supreme. Being. Contain. Yourself."
- (To Demiurge about Ainz's future heir): "...Oh. How. Wonderful. What. A Glorious. Sight... Uncle... Uncle..."
- (To Demiurge): "Ah. What. A. Beautiful. Sight... Truly. it. Was. Everything. I. Have. Ever. Dreamed. Of."
- (To Demiurge): "...Demiurge. You. Dare. Ask. Albedo. To. Resign. The. Position. Of. Guardian. Overseer. Which. The. Supreme. Beings. Gave. Her? This. Is. Treachery!"
- (To Albedo and Demiurge): "I. Mentioned. It. Before. But. The. Entire. Fight. Has. Gone. As. Ainz-sama. Planned. I. Am. In, Awe. Of. His. Prowess."
- (To Albedo and Demiurge): "...His. Extraordinary Ability. As. A. Magic. Caster... No. As. A. Combatant. Is. Truly. An. Inspiration."
- (To Demiurge): "I. Hope. You. Can. Help. Me. Think. Of. Something. If. This. Carries. On. We. Will. Be. Defeated. I. Can. Accept. A. Personal. Defeat. But. I. Cannot. Allow. The. Great. Tomb. Of. Nazarick. — The. Supreme. Beings. — To. Be. Disgraced. In. Such. A. Way."
- (To himself): "They. Will. Never. Forgive. Me. If. Things. Get. That. Bad. Not. Even. My. Life. Will. Be. Enough. To. Atone."
- (To himself): "If. The. Master. Is. Disappointed. And. Leaves. Like. The. Other. Supreme. Beings. What. Should. I. Do..."
- (To himself): "No. That. Cannot. Be. Ainz-sama. Would. Never... Abandon. Us."
- (To himself): "How. Could. Such. A. Merciful. Master. Abandon. Us?"
- (To Ainz): "I. Am. Deeply. Remorseful. For. My. Inability. To. Achieve. Victory. Even. After. Receiving. Command. Of. My. Own. Troops. And. For. The. Loss. Of. The. Elder. Lich. Commander. That. You. Personally. Made. Ainz-sama."
- (To Ainz): "I. Underestimated. The. Lizardmen. I. Should. Have. Been. More. Careful."
- (To Ainz): "Yes. I. Did. Not. Have. Enough. Information. From. This. Battle. I. Learned. That. My. Chances. Of. Victory. Would. Be. Slim. If. I. Did. Not. Know. The. Enemy’s. Strength. And. The. Terrain."
- (To Ainz about the Lizardmen): "Yes! In. The. Future. They. Might. Breed. Mighty. Warriors. Therefore. It. Would. Be. A Shame. To. Wipe. Them. All. Out. Right. Now. Your. Servant. Submits. That. It. Would. Be. Better. To. Instill. A. Strong. Sense. Of. Loyalty. Among. Them. So. That. When. Stronger. Lizardmen. Appear. We. Can. Take. Them. As. Minions."
- (To the Lizardmen): "This. Might. Be. Rude. To. Warriors. Who. Are. Ready. To. Give. Their. Lives. But. I. Must. Inform. You. That. My. Side. Of. These. Pillars. Will. Be. Your. Grave. Any. Who. Cross. It. Shall. Die."
- (To Albedo): "The. Opponent. Desires. To. Have. A. Battle. Of. Wits. With. Ainz. Sama, If. We. Rejected. Him. Outright. It. Would. Reflect. Negatively. On. The. People's. Impression. Of. Ainz. Sama's. Magnanimity."