During the battle in the Infinity Castle, just as Yushiro was about to send Muzan and everyone outside of the castle. Portals appeared and led Muzan and the rest of the current Demon Slayers to be sent somewhere else besides above the ground. Though a few other friends and foes would soon join a group of misfit pirates *Spoilers for Demon Slayer Manga and One Piece Manga* One Piece x Demon Slayer: Kimetsu No Yaiba One Piece is owned by Eiichiro Oda and Toei Animations, please support them. Demon Slayer is owned by Koyoharu Gotoge and Studio Ufotable, please support the official release.