The Death We Seek

A Codex Bound in Skin and Writ in Blood [Chapter 10]

The days following him having caught Aurora in the basement, presumably undergone some kind of forced long-range teleportation, were tedious to say the least. He couldn't tell Rias or her peerage anything, and instead reported directly to Sirzechs. His parents had to undergo memory-erasing techniques to make them forget about his use of magics.

Issei had recovered well. According to Elsha and Ajuka, his brain had fully healed and was operating even better than it was before. They each theorised that it was because of his repeated contact with higher dimensional constructs. His body had indirectly prepared him for the next encounter better; a vivid demonstration of evolution in a microscopically smaller timeframe.  Regardless, he was still suffering minor hallucinations with no known cause.

Sirzechs and Ajuka were more than extremely concerned about what he had done and seen. They practically demanded to go into his basement to see for themselves, accompanied by an army, but Issei's unrelenting refusals had convinced them.

He had potentially the strongest mental barrier of any living creature, thanks to his past traumas and experiences combined with his shared mind between Belzard, Elsha and Ddraig. They took minor portions of each attack on his mind and greatly reduced the impact, and yet he was helpless in front of the concepts. If the devils saw what he had seen, it's not unlikely that they would succumb to madness. Elsha had also speculated that he was so vulnerable to the following psychological assault due to his previous contact with Infinity, alongside the lack of Ddraig and the others to take a load from him.

He was already lucky enough to come out of it as unscathed as he was. If Ophis didn't want him to live for its entertainment, it could have ended his existence without a thought. The Unknown Concept, as Ajuka had named it, seemingly didn't want to kill him outright either. Or it couldn't, for another unknown reason.

Issei had to teach the beholders of the Thrones of Hell what concepts were. Previously, he had learned from Odin and the Ouroboros. Odin was determined to take the knowledge to his deathbed, and only told him the nature of the strongest powers while in a puddle of his ichor. Ophis had told him in passing during some of the first attacks of the mirages. It was a joint assault with the Khaos Brigade, its very own terrorist force, to weaken the defences of the Grigori and the Underworld while the machines attacked Heaven simultaneously.

Since then, Issei had used and abused any vow he could make with the World. The Concept of Life operated on an exchange basis; it would allow any being with life to make a pact with itself in exchange for power and a sacrifice made. The sacrifice could be something as simple as a statement such as "I will only use my left arm for this attack", and the resulting attack would be significantly empowered.

Yet another reason why he was the last one standing.

He did not tell them about the details of each concept, only their existence and nature. It almost immediately reminded Ajuka of the Trihexa and, upon his resulting question, was met with an "I don't know" from Issei. Concepts weren't quite alive and, as the original God had proclaimed once, the Beast of the Apocalypse was an extant form of life. A twisting malformed life form, with a poorly-defined nature, but a life nonetheless.

Another thought came to the Super-Class devil, that of the necronomicon. It was a book bound in human skin containing countless knowledge of magics and stories of the Old Ones. Ancient Outer Gods that, while can be summoned, spread madness and manic hysteria amongst their ritualistic followers. The necronomicon was passed to the Thrones of Hell from the Original Lucifer and kept safe within their reach. Issei didn't make the connection between it and the concepts.

Ajuka, however, was possibly the most intelligent being alive. He had more knowledge than most gods on their own domains, and could reduce any spell into simplified mathematical equations before rewriting it and nullifying it. He was proposing that the book had knowledge on numerous forgotten gods, and the identity of the Unknown Concept may lie within.