Underfell

Cross!Sans staring towards this universe's Snowdin.

The **Underfell** universe refers to 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 to reign havoc on Underfell as part of a deal he made with Nightmare!Sans in order to 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.

## History

In the midst of a Pacifist Timeline in Underfell, Nightmare and his newly-found accomplice Cross break into the alternate universe. Explaining the premise of this alternate universe, Nightmare tells Cross to "have fun", as Cross begins formulating his plan to conquer the alternate universe as his own.

This involves marking any corners around Snowdin with an X, making the inhabitants anguish and annoyed. This gains the attention of Sans, who berates Cross for his foolish plot and tries to defend Snowdin as much as possible. Cross gains the upper hand thanks to X-Event!Chara's X-Event ability, and is nearly successful in killing Sans.

Ink and Classic!Sans arrive to witness the destruction that Cross had forced upon Snowdin. As soon as Cross notices their appearance and their refusal to accept Underfell as his own world, Cross infects Sans and forces him to fight the two together. While in the midst of their brawl, Snowdin is entirely taken from Underfell, being implemented in the then-empty XTale.

Abruptly, after trapping the infected Sans, Classic!Sans confronts Cross over the premise of XTale, the powers of the X-Event, and XGaster. This leaves Cross in a state of inability to continue pursuing the two, and with some distraction by Sans with his pink slipper, it leaves plenty of time for Sans to be freed of Cross' control, and strike revenge against Cross. Unbeknownst that Cross had already taken half of Classic!Sans' soul, Sans nearly obviates Cross, who manages to flee thanks to X-Event!Chara.

With the battle now over, the three look around to see an empty Snowdin. Sans accuses Ink of deliberating destroying his Snowdin, but their arguments are cut short when Error!Sans steals Ink away. Classic!Sans runs off as soon as the Underfell casts swarmed in, and Sans pursues him after realizing his brother, Papyrus, caught wind of his actions from one of Alphys' cameras.

Classic!Sans, while taking a break in the middle of a echo flower field, gets an advice from Core!Frisk on how to take out Cross. After Ink leaves the eyes of Error, the three of them awaited as Dream!Sans spies on Cross' next move. Once the three know he's going to Underswap, the three leave the universe in pursuit of Cross.

Cross' mess with Underfell did not go unnoticed with XGaster. He asks his assistant, XAlphys, to place any timelines affected by the X-Event to be quarantined. This includes the already effected Pacifist Timeline that Cross slipped into. Core!Frisk would soon be the subject to witness XAlphys' ordering of XTale!Mettaton's purge of the Underfell inhabitants, including a dissentious King Asgore. The quarantine causes everything of Underfell that was extracted into XTale to dissipate, as with the Underswap characters.

Due to the quarantine efforts by XAlphys and XGaster, this and Underswap are the only two universes not to be destroyed by Error!Sans.

## Characters

### Underfell!Sans

*See Underfell!Sans*

A very selfish, aggressive version of Sans, known for his abuse of curse words and his demeaning attitude. Joins up with Sans and Ink to flee from his brother until he meets his demise by XGaster.