Two sentence villainy.

Izuku had a long day in middle school, getting beaten up by Bakugo and mocked. So when he returned home, he decided to scroll through the two sentence horror subreddit and decided to make a post there. Not knowing that what he wrote down is going to happen to someone in the world within an hour. Nor does he know that every other two sentence horror he writes will also happen to someone.

Basically Izuku's quirk here is kinda like the Death Note, except it functions by making whatever two sentence horror he writes real while targeting a random person. For Izuku to target someone using his quirk, he'd have to write their first and last name, or whatever they go by so in Zen Shigaraki's case it'd be AFO. Also Izuku doesn't realize that he has a quirk and what he is doing.