The basic, spoiler-free gist of it is that a multiverse-wide apocalypse happened and fragments from the different universes were saved and kind of smashed together into one big world called Battleworld. In the story itself there's laws and walls and all that to keep the different realms of Battleworld from mingling (for the most part, of course) but I feel like that could be something that'd be done differently in an RP to expedite all the alternate universe misadventures.