Underfell is an aesthetic AU created by an unknown author(s) that was put up on a blog, abandoned, and then taken up by the Underfell fanbase.
About the AU
The premise of Underfell was that characters were first and foremost created as an aesthetic design about what an alternate universe would look like if the characters' moralities were reversed. In other words, kind monsters were turned evil, and evil monsters turned good. There was never an officially established canon, other than the design of all the main characters themselves.
There is no officially-established plot. However, the original blog owners eventually left the fans of it to try and come up with it themselves. There are various different interpretations of the AU among the fanbase, each with their own plot and designs. There are many different stories told throughout the community, each trying to give their own variation and spin on the story. Among the few of them, they include the following: