This happens after Class 1A start working as Heroes. But someone or something started prowling the streets at night... (I have only seen mha until season 3 so don't blame me if it's inaccurate ;-;) (Villain Tanjiro AU don't judge hehe)
Tanjiro Kanado's life has gone to hell. His whole family was brutally murder and his sister has turned into a demon. However, she still has humanity and continues to love him. Now Tanjiro needs to learn to become a demon slayer to cure his sister, however, every hero in Japan is looking for them so both are on the run and train in a hidden world that not even the heroes know.
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.