- "Fifth Floor Guardian Cocytus, at your command."
- — Cocytus' Introduction
Having an enormous body size of 2.5m, 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. As 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 relayed questions on who Cocytus is the friendliest within 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 that wants is for Ainz to have an heir so that the future of the Guardians will be secure and the chance to train said heir.
The Bloody Valkyrie Arc
- Main article: The Bloody Valkyrie Arc
Cocytus was 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 was among the judges of the auditions in selecting the person for Juliet. He was in disbelief that Demiurge supported the idea for a competition between the two Floor Guardians, Albedo and Shalltear. Later on, he was training on the 5th Floor and was approached by Shalltear. Cocytus knowing something suspicious was up, attempted to rebuff her. His suspicion was true when she tried to bribe him to vote for her in the auditions. However, when she offered the role of Tybalt to him, for a chance to exchange blades with Ainz, he was tempted by the idea. Cocytus, though being an honorable warrior, refused and told her to leave. During the audition, Cocytus was moved by Albedo's performance which resulted in him voting for her.
The Sealed Evil Tree Arc
- Main article: The Sealed Evil Tree Arc
Cocytus was summoned to the center of the Tob Forest to fight against Zy'tl Q'ae. He was ordered to fight as a team with the Floor Guardians, however, he kept forgetting and twice attacked the treant with overwhelming force.
The Lizard Man Heroes Arc
- Main article: The Lizard Man Heroes Arc
Cocytus was ordered to annihilate the lizardmen and commanded 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 is 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 left 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 which Ainz agreed.
To redeem his failure, Cocytus was ordered by Ainz to do battle alone against the lizardman heroes lead by Shasuryu Shasha, Zaryusu Shasha, and Zenberu Gugu. After a quick battle, Cocytus kills all of them by 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 went out to subjugate the Toadmen in the northern part of the Great Lake. Cocytus returns to Nazarick and joins Ainz, Demiurge, and Mare in 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 was brought up. Cocytus said clothes were unnecessary for him, earning him a tease from Aura that he was naked. He later suggests asking Ainz for weapons as he was interested in adding to his collection and also to provide new equipment for the lizardmen under his charge. When Aura wanted to have Rororo, Cocytus furiously denied it, stating he would not allow his charges' property be taken and would even bring the issue before Ainz's attention should Aura escalate it. He, however, considered 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 of 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 he requested a voucher to fight with Ainz as his reward. In addition, he formally requested as the Overseer of the lizardman to prevent the seizure of Rororo, by Aura which Ainz deemed reasonable.
Like the others, he was given three gold coins to be used as his salary by Ainz. In the next meeting, he presents 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 he gave them orders them to spend. Finding little interest in the goods offered in E-Rantel, the Floor Guardians sought advice from Sebas Tian and Narberal Gamma. Noticing the fan goods of Blue Rose, it prompted 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 at the Throne Room, where Ainz Ooal Gown welcomes the imperial delegation from the Empire. Later, he and the rest of his fellow Floor Guardians held a meeting with Ainz, where he discussed his impressions of the Empire and its leader. At the same time, he suggested 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 came to the Great Lake to talk with Zenberu on the Dwarf Kingdom. He was clearly outraged when the lizardman was hesitant to answer Ainz's questions but was held back at the order of his liege. After misunderstandings were cleared up, Ainz informed him of his intentions to go to the Dwarf Kingdom and forge diplomatic relations. Also, Cocytus was 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 impressed the lizardmen of Ainz's power over him.
Before setting out, Cocytus was wary of the potential dangers and wanted to personally escort the Sorcerer King during his journey to the Dwarf Kingdom. However, Ainz convinced him that should Cocytus be in peril or negotiations would deteriorate, he would be the first one to be summoned to his side.
Nazarick Mythology Arc
- Main article: Nazarick Mythology Arc
Cocytus attended 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 was 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's 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 told Ainz about Cocytus requesting that they record their battle with him. This way, Cocytus believed that everyone in the Great Tomb of Nazarick can learn from the battle which his master permitted the group to do.
- 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 was 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."