- "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 with 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 and respects anyone with a fighting spirit and the heart of a warrior. As a Floor Guardian, he's 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.
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 in allowing outsiders join Nazarick. Though he takes pride in his position as Nazarick's defender, he is somewhat envious of his fellow Floor Guardians. He likens himself as the 'sword' of Ainz Ooal Gown, that executes his master's wishes and thinks the only way he could prove his worth to Ainz is by actually defeating the invading forces that would be a threat to Nazarick. Cocytus isn't 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 about all the Supreme Beings' Divine Weapons.
The Undead King Arc
- Main article: The Undead King Arc
When Momonga calls for the Floor Guardians to the sixth 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 Bloody Valkyrie Arc
- Main article: The Bloody Valkyrie Arc
Cocytus was ordered to restrain Demiurge from interfering the fight between Ainz and Shalltear. When Ainz emerges victorious from his battle, Cocytus 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 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, he requests advice from Demiurge, who he considers to be his rival and doesn't want to asked for help, but has no 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 fights 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 back to Nazarick and joined Ainz, Demiurge and Mare to the Spa Resort before the interruption of the female Floor Guardian who are attacked by the Lion Golems.
The Dark Hero's Story Arc
- Main article: The Dark Hero's Story Arc
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, he is asked for his opinion of his decision to allow Tuare to join Nazarick of which he has no objections to.
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 welcomed the imperial delegation from the Empire. Later, they held a meeting with Ainz and the rest of the his fellow Floor Guardians, where he discussed his impressions of the Empire and its leader. At the same time, he suggested on addressing Ainz Ooal Gown as the Sorcerer King for the newly founded nation.
The Craftsman of Dwarf Arc
- Main article: The Craftsman of Dwarf Arc
Cocytus greeted Ainz when he came to the Great Lake to talks 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 his intentions to go to the Dwarf Kingdom and forge diplomatic relations. Cocytus allowed his master to use 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 Kindom. However, Ainz convinced him that should he be in peril or negotiations would deteriorate, Cocytus would be the first one to be summoned to his side.
- 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 9th floor to the 10th floor. He commands a troop of armored beetle warriors and 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."