Sekai Oni x Mob Psycho 100 is a crossover chapter of the Mob Psycho 100 manga series with the Sekai Oni manga series. It was illustrated by Eru Okabe.
Shinonome comes to Reigen for help, telling him she sees strange things in mirrors and that she couldn't sleep. Reigen then says that paranormal and mirrors are closely related and, with Shinonome's prompting, he begins telling her all sorts of myths and lore about paranormal activity involving mirrors. Satisfied, Shinonome goes to leave when Mob arrives with some mirrors. He encounters Shinonome but Reigen tells him she wasn't a customer.
Mob then asks Shinonome what she sees in the mirror and she reveals all the things she sees in the mirror when people went in front of it. After Reigen's reflection is revealed, he concludes the mirror to be possessed and proceeds to 'exorcise' it with a salt-covered punch. Sending shards of glass flying everywhere, Mob tries to stop them from hitting Shinonome with his psychic power but she ends up getting ten shards of glass embedded in her face.