- "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. 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. Unlike his isolationist and xenophobic colleagues, he is more accepting of allowing outsiders to join Nazarick.
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.
Cocytus was created by Warrior Takemikazuchi as the Floor Guardian of the 5th Floor of the Great Tomb of Nazarick. He was programmed to possess knowledge on all of the Supreme Beings' Divine Weapons.
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. He stops the argument between Aura and Shalltear in front of their master. After all of the Floor Guardians arrive, he pays his respects with the rest of them to Momonga.
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. As for his desires, it would be to spar with Sebas Tian, Albedo, and even Ainz if permitted. Another wish is that he wants Ainz to have an heir so that the future of the Guardians will be secure and wants the chance to train said heir.
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. When 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.
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 twice attacks the treant with overwhelming force.
The Lizard Man Heroes Arc
- Main article: The Lizard Man Heroes Arc
Cocytus is ordered to annihilate the lizardmen and commands the undead army of Nazarick. Seeing his army being defeated by the lizardmen, Cocytus requests advice from Demiurge, who he considers to be his rival and does not want to ask for help, but has no other choice. He deploys Iguva=41 to the battlefield. Seeing Iguva=41 being destroyed and his army being defeated by the lizardmen, he realizes that his mission to annihilate the lizardmen has failed. 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. Still, he is deeply impressed by the lizardmen's victory over him. After his failure, Cocytus asks Ainz if they could spare the lizardmen and make them a part of Nazarick, to which Ainz agrees.
To redeem his failure, Cocytus is ordered by Ainz to do battle alone against the lizardman heroes led by Shasuryu Shasha, Zaryusu Shasha, and Zenberu Gugu. After a quick battle, Cocytus kills all of them with his hands. Because Cocytus emerges victorious from his battle against the lizardman, he is now in charge of the occupation of the lizardman tribes.
The Two Leaders Arc
- Main article: The Two Leaders Arc
Cocytus and the lizardmen head out to subjugate the Toadmen in the northern part of the Great Lake. Cocytus returns to Nazarick and joins Ainz, Demiurge, and Mare in the Spa Resort Nazarick before the interruption of the female Floor Guardians, who are attacked by the Lion Golem.
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.
The Men in the Kingdom Arc
- Main article: The Men in the Kingdom Arc
Cocytus is present with Ainz as a witness when they are questioning Sebas about his possible betrayal. After everything has been cleared up, Cocytus is asked for his opinion concerning his decision to allow Tuare to join Nazarick, which he has no objections to.
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 Magic Caster of Destroy Arc
- Main article: The Magic Caster of Destroy Arc
Cocytus is present in the Throne Room, where 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 Empire and its leader. At the same time, he suggests addressing Ainz Ooal Gown as the Sorcerer King for their newly founded nation.
The Craftsman of Dwarf Arc
- Main article: The Craftsman of Dwarf Arc
Cocytus greets Ainz when he comes to the Great Lake to talk with Zenberu on the Dwarf Kingdom. He is clearly outraged when the lizardman is hesitant to answer Ainz's questions, but is held back at the order of his liege. After 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 months ago. In a way, his action impress the lizardmen of Ainz's power over him.
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. However, Ainz convinces him that should Cocytus be in peril or negotiations deteriorate, he will be the first one to be summoned to his side.
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.
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 was present when Shalltear brought along Hilma Cygnaeus to present his master 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, approved of the punishment Ainz has suggested for Hilma Cygnaeus to do. When that matter was 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 was the second person after Aura to question the strategy of their operation. Cocytus wanted to know why they didn't make use of the other noblemen besides Philip during the beginning of the operation. Demiurge answers Cocytus's question, stating that they did recruited someone like that already which was through blackmailing Philip's father, Baron Montserrat. It wasn't just that nobleman alone, but Demiurge also supports Cocytus's way of thinking, choosing to spare an expendable portion of the nobility who were at least somewhat capable and worthy for Ainz to rule over.
When Demiurge asked 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 had contributed to the campaign by destroying a city. He has also employ a tactic to successfully conquered several cities without problem using an undead army borrowed from his master. His work was commented by Ainz during a meeting with the Floor Guardians in Nazarick.
Alongside his master and his fellow Floor Guardians, Cocytus was among those besides Demiurge and Cocytus to discover the reason why Ainz did not sent so many forces in the city of E-Naüru. This led to his colleague Demiurge praising him and Shalltear for understanding the reason behind why their master made the decision he did by sending in weak forces.
- 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.
- (To Momonga): "More powerful and stronger than any Guardians. Supreme ruler of the Great Tomb of Nazarick."
- (To Aura): "A sword does not think, swung to the will of its master, it only cuts."
- (To the Lizardmen): "This might be rude for the warriors who came with the resolve to die, but know this, only death awaits all who cross beyond these ice pillars."