Underfell is an alternate universe created by multiple owners and is currently adopted by the Undertale fandom. The universe is influenced by the idealism of "kill or be killed", where even the most innocent characters in Undertale, like Toriel, are transformed to be rude and merciless.
In Underverse, Cross!Sans is sent here by Nightmare!Sans as part of their deal for which he must harm one's mental self for Nightmare's own gamble, so that he can traverse multiple AUs. During Underverse 0.2, he fights off Underfell!Sans and defeats him, using him as a puppet against Sans and Ink!Sans. While Cross is defeated, he was able to grab the entirety of this universe's Snowdin, and Underfell!Sans was forced to join the cast to run away from his brother after what he has saw.
- See Underfell!Sans