You were a student at UA training to be a hero and you constantly lied about your quirk because your quirk was Space-Time Manipulation and you could alter the fabrics of reality & the very existence itself, allowing one to erase existences and not just the flow of time and the fabric of space, but also reorganize, alter and erase historical events/timelines/time streams and locations/realities and combine universes. One day you were fighting a villain when you accidentally used Soace-Time Manipulation and combined two universes. You combined your universe and the demon slayer universe. Your class, teachers, and villains were confused by the sudden appearance of these new people.

the Demon Slayers and Demons got up

Tanjiro: Where are we Zenitsu: looks around and sees the demons and screams Inosuke: sees Bakugo YOU THERE, YOU WANNA FIGHT!?! Bakugo: IS THAT A CHALLENGE!?! Denki: Who are you people? Rengoku: I'm Kyojuro Rengoku the flame Hashira Tengen: Tengen Uzui the sound Hashira Shinobu: Shinobu Kocho Mitsuri: What is this place Obanai: No idea Gyomei: this is confusing Muichiro: what was I doing Giyu: silent Sanemi: angry Genya: Confused Kanao: silent Nezuko: in box Muzan: talking to Shigaraki and Dabi All Might: stands there Aizawa: What is with those uniforms Tanjiro: We're demon slayers Izuku: So you kill demons? Iida: what's that like? Ochako: Is it dangerous? Momo: let's give them space Todoroki: Becomes friends with Giyu by staring at him