[A/N:This is the flavour text which is cannon to the NPC Ryoumen Sukuna. This will be updated slowly as it is gonna be a couple thousand words of bullshit backstory.]
Ryoumen Sukuna. an NPC created by every member of Ainz Ooal Gown, except for Tabula Smaragdina. The NPC is the youngest in Nazarick, and was created as a result of Tabula creating Rubedo. After her creation, Tabula joked that it was the greatest thing the guild was ever going to make. Although there was no ill will behind this statement, it set every single one of his guild mates off. And thus, a quest was created involving the creation of an NPC better than Rubedo. Every member contributed. Touch me and Ulbert as well as Punitto Moe, Takemikazuchi and Peroronchino designed the build, while Nishikienrai and Amanomahitotsu researched the weapon. The others decided to collect the items that would be required and Momonga even shared how he had broken the cap on his MP limit. The members of Ainz Ooal Gown researched for hours, testing builds, and optimising levels, until eventually, a build that could completely best Rubedo with a 99.8% accuracy was found.
Even then, the build was deemed unfeasible, as it would require several job classes that would be normally blocked out. For example, the Job classes 'weapon master' and 'Archmage', both were mutually exclusive and couldn't be put on a single build. But they were needed in order to get the battle god Job class, which was crucial for the build's stability. Thus, they decided that they would hunt down a world item to remedy that.
The "Ouroboros golem core" was a world item that could remove all restrictions placed on NPC creation, as well as provide several buffs such as immunity to mental control spells, a 15% buff on special attack, range extension on AOE attacks of upto 20% etc. Combined with the caloric stone, the result would surly be one of the best NPCs in Yggdrasil as a whole.
The problem was, the item was held by a top 10 ranked guild, the "Blade Runners" and would be near unattainable without heavy consequences. Still, much to Tabula's chagrin Ainz Ooal Gown declared a guild war over that item. Eventually, they managed to win, adding a 13th world item to their treasury, albeit for a short period of time.
Their efforts didn't end here, as they still had to hunt all the weapons and armour required to run that build successfully, but eventually, they succeeded, creating the strongest NPC in Nazarick, which even Tabula acknowledged.
Then, they began to think up a backstory for the new NPC as a proper RP guild would. They also created a design for him, heavily inspired by a popular manga created nearly a century ago. Eventually, with the help of tabula, who was roped in due to losing a bet he didn't remember making, created this backstory for him.
[Ryoumen Sukuna, "the fallen one" of Nazarick is the most evil creature held in its walls. It takes great pleasure in the suffering of others. not caring for life beyond what fun he can have at its expense. He is a clever being, albeit in a way different from Albedo the guardian overseer, or Demiurge the chief strategist. His intellect is solely focused on bringing death to others, and due to this, his intellect is merely average in other matters not related to carnage.
Despite his nature, he almost always acts in a calm, reserved manner, giving an aloof, slightly regal air. Of course this is merely a front, to get his prey into its comfort zone, before he consumes it.
He is very blood-thirsty, constantly searching for another fight, just so he can humiliate and crush his opponents in the most complete manner. He loves exploring the unknown as it is the only thing that can distract him from his thirst for battle.
He is also an avid poet and also appreciates looking at flowers. He often writes poems about the sixth and fourth floors, as he patrols them. He also has a taste for human flesh as it is his favourite delicacy, especially that of infants.
He is tasked with constantly patrolling all of Nazarick, in order to find intruders and surprise them with a boss fight when they least expect it. This unique role gets him certain powers within the hierarchy of Nazarick as he is not managed by guardian overseer Albedo and instead acts as a separate body from the typical guardians. Speaking of which, he does not get along with Albedo well. There is mutual respect on both sides, but the Albedo always wishes to get a one-up on him, due to the fact that he was created to be better than her and her sisters. Of course, he is not aware of this fact and just dislikes her because she constantly tries to put him down.
As for the other guardians, they respect him as someone who is the most reliable NPC in Nazarick. However he keeps his distance from them, only giving them the respect they deserve as fellow guardians. He also shows interest in going outside of Nazarick, wanting to explore the world and see what kinds of mayhem he can truly cause there. there is no area in Nazarick he is not aware of, so that he can travel where ever he wishes, so that no enemy can escape. There is however one exception two exceptions to this, the innermost sanctum of the treasury and the women's bath on the ninth floor, for obvious reasons.
Also, although he rarely interacts with the floor guardians, the area guardians are an entirely different matter. He is allowed to visit them, and thus he has built a good relationship with them. This includes writing Haikus with Aureole Omega, cleaning artefacts with Pandora's actor and some tea with Pestonya and he maids. Even Kyouhuko has a pleasant relationship with him, as he is someone who doesn't feel disgusted in his domain.
He also loves weapons and magic, obsessing over finding new weapons and finding different ways to use them. This is a thing which he and Pandora's actor bond over.]
He was also the one that caused the 1800 player raid on Nazarick to fail, as the players ran into him while traversing through the eighth floor. the players would have failed anyways, but his presence was the final nail in their coffin.