Please note that there is a full version of this game as well as a very limited demo. There is even Trainertale, an AU based on Pokemon and Undertale. Sans Simulator is a fan-made free battle game, while Underfell: Revolution, takes place in two locations.
Similar AU spin-offs of Undertale include Undertale: The Second Chance, Underswap, Deltarune, and Unitale. Conversely, other characters that were good in the original Undertale are now bad in Underfell. In Underfell, however, the friendly monsters have been replaced with malicious versions who want to do the player harm.
For those new to the Undertale world, the original game featured good-natured monsters who didn't want to hurt the protagonist.
