A mysterious man called Callypso announces a 1v1 tournaments with three sign ups. Ultimate prize is ANY wish.

*Free sign-ups.

Basically Twisted Metal combined with Street Fighter.

This occurs right before the final war. Fractions halt their war as rules of the tournaments disallow fighting between participants until the tournament is over. Aside from that, anything goes. Everyone is freaked about a guy who can seemingly grant wishes out of nowhere but decides to partake just in case.

Shigaraki manages to win and wishes for destruction of everyone who ever wronged him. Callypso, being a massive troll, causes Tomura to decay. He explains that AFO was the only person who wronged him, and he is currently within the boy's body. Izuku is silently glad that he dodged a bullet.