(well i was gonna make it but okie 0.0) so basically, there are a lot of aus and most are the OG ones (its limited because otherwise it would get chaotic) like Underswap, UnderFell, Undertale, Fellswap, Outertale, ect. and its connected to one big timeline called "World Tree". In each AU its a little different, but basically there's a kingdom of monsters and a kingdom of humans. They are either at war or have peace (depending on the AU) and they both have dragons that come from the World Tree