Note: The description of the question contains some crucial spoilers, so I decided to put the whole question in a spoiler tag just in case I don't spoil for some users that haven't watched the anime or read the manga.
@Memor-X it's an alternate universe explored in one of the events in the mobile game, XDU
this (official) short set of comics showcases three different AUs (who are hinted to reappear in an upcoming event? I'm not sure, I don't really play the game :3) twitter.com/YameteTomete/status/1350446679313571840
