User blog comment:SuperBearNeo X/List of Allowed Undertale AUs/@comment-38451037-20190421214625

Underverse was techniaclly a mishmash of multiple well established AUs (CORE!Tale, ERROR!tale, INK!tale, etc.) with X-tale being the prime focus.

Basically, everything that have a huge following in the AU undertale community (to the point that they're part of the Fanon Multiversal Lore).