Muzan Kibutsuji was dead- dying at the hands of The Demon Slayers after warring with them for over 1000 Years. When he died, he knew full well that he was never going to reincarnate... or so he believed! After an untold amount of time in the void, a strange voice gave him a choice, remain in the void for the rest of time? Or... become reborn. He chose the latter.
After being told that he can't be a hero by All Might, Izuku ends up in a state of depression. One night after a walk he bumps into a pale man who turns out to be the leader of the demons named Muzan. Izuku while fleeing from the demons is saved by the Hashira, who work in the shadows of the hero society. Now the powerful samurai will teach Izuku how to become a strong hero.
Muzan Kibutsugi was dead, and would never reincarnate due to his status as a Demon and because of his actions. Or at least, that was what everyone thought. While Muzan would never get the chance to truly live again, he could always make sure his legacy carried on in an event of a rebirth... (OP!Izuku, MorallyGrey!Izuku, Demon!Izuku, HeavyCanonDivergence)