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Synonyms: Underfell AU, UF
See also: Undertale, shared universe
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Underfell is an "evil" dark alternate universe of Undertale where a number of elements, particularly the monster characters, are changed and portrayed as much more hostile than in canon. Some fans refer to these characters with 'Nega' as a prefix to differentiate from canon.[1] Many Undertale fans took inspiration from the original works and created a shared universe.

This article or section needs expansion.


The Underfell's wiki page on Undertale's AU wiki credits this AU as having started on October 17, 2015,[1] and according to the Underfell wiki it was the first Undertale AU ever created.[2] It was created by by Fella/Vic who posted fanart and character designs on the Tumblr blog underfell, though the blog had more than one moderator, some of which were minors.[3] Toriel was the first character who had an Underfell version created before it expanded to other characters.

The AU, during its beginnings, recieved some backlash from other Undertale fans for "ruining" the meaning of the game.[4] At some point the Underfell blog had been shut down before coming back online. However, after this re-emergence most of its early content had been lost.[2] The Wayback Machine is needed to view older entries of the blog.


While Fella was the original creator of the Underfell designs and Tumblr page, he gave very little to the canon of the characters and AU, preferring that fans of the AU create their own stories. This means while there are some basic ideas that most fans accept as 'canon,' the AU is still largely up to interpretation.

Fella does not approve of NSFW content in Underfell, but provided a separate tag for it anyways, called "Underfrick."[2] Many old posts found on the Wayback Machine remind fans to "keep it pg13," however given how large and popular the AU has gotten since 2015 this is nearly impossible to prevent.

About Underfell


In Underfell the monster characters are changed and portrayed as much more hostile than in canon, with the exception of Flowey.

  • Frisk: Much less enthusiastic and energetic than canon Frisk. Because Frisk's behavior is mostly up to the player in the game, Frisk's Underfell personality is more easily up to interpretation. However, usually in Underfell Frisk and Flowey are depicted as "pacifist run characters in a genocide run game." Frisk has black, pink, and blue stripes on shirt.
  • Flowey: Generally portrayed as friendly and helpful. Sometimes characterized as being wilted, such as in the first image created for this AU by the Underfell Tumblr.
  • Asriel: Before becoming Flowey Asriel was a normal goat monster. He was defensive and nervous about Chara before they eventually became good friends. However, on the Underfell wiki it's noted that Asriel would still bate humans to be killed by monsters. He became less evil when he turned into Flowey.
  • Chara: Prefers monsters over humans and can see both the good and bad of humanity. Asgore poisoned them.
  • Toriel: Simply an "evil" version of Toriel who wants to kill the player/Frisk, calling them an "evil child." Bad at baking but makes good meat pies. Characterized by torn clothes, yellow eyes, and claws.
  • Asgore: More buff and evil. In this AU he is the one to state that the world is "kill or be killed." Asgore poisoned Chara.
  • Sans: Often nicknamed "Mustard" in meet-your-AU-self fics and roleplays. Has the same role as canon Sans but darker and edgier, with emphasis on a strained relationship with Papyrus. Often portrayed as some degree of traumatized or cowardly, having a tendency of unhealthy attachment to Papyrus or Frisk, and being into sadomasochism in some way. Underfell Sans can be characterized by one red eye in his left eye socket and a golden tooth, and sweats often.
  • Papyrus: A darker and edgier version of canon Papyrus, generally portrayed as actually bloodthirsty towards Frisk and much more harshly critical of Sans, sometimes even being physical abusive. He is more successful in his tasks than Undyne is, though she is still the captain of the Royal Guard. Underfell Papyrus can be characterized by a jagged mouth and two scars over his left eye.
  • Undyne: She remains the captain of the Royal Guard. She has a more calm personality but has little change other than this. Can be characterized by matte lipstick and usually wears latex.
  • Alphys: Arrogant with little to no empathy. She is also very confident in herself. Characterized by swirled glasses, an upright posture, and a jagged skirt.
  • Mettaton: Characterized by four arms and a perma-smile.

Spin Offs

Like many AUs in Undertale, Underfell has also produced its own AUs.


Flowerfell is an AU based on two other AUs: Underfell and Flowertale.[5] It involves floral body horror like in Flowertale, as a flower grows on Frisk's body every time they die, but makes use of Underfell characterizations. In this AU Sans, Papyrus, and Frisk are all the major characters that the authors put focus on. Sans is 7ft tall, Papyrus is 10'8", and Frisk is written as an adult at 18 years old.[6] It began on Jan 4, 2016 by a user named Siviosanei and was popularized through fanfiction such as Overgrowth. A few of Siviosanei's friends also contributed to this AU.

Flowerfell was subject to repeated incidences of art theft and unauthorized reposting to other websites, such as AO3. Some of the creators were subjected to harassment and threats, as well. After some time the project's original creator put the whole series on hiatus, and made the most plagiarized works in the series available only via her Patreon to try to avoid further art theft. Authorized translations of those works are only accessible via paywall as well. However, Overgrowth can still be listened to in three parts on Youtube here, even after being removed from AO3 due to plagiarism and harassment.

The original creator of the AU has deleted all posts on the Tumblr account she used at the time and has moved to a new account because of the harassment she experienced.


Swapfell is a combination of the alternate universes Underfell and Underswap.

Example Works

Examples Wanted: Editors are encouraged to add more examples or a wider variety of examples.

Links & Resources



  1. ^ a b Undertale-AU Fandom Wiki (Accessed 4/17/2020)
  2. ^ a b c Underfell Wiki (Accessed 4/17/2020)
  3. ^ Whats This Bad Boy, Tumblr Oct 2015 (Accessed 4/17/2020)
  4. ^ Tumblr, Oct 2015 (Accessed 4/27/2020)
  5. ^ Flowertale, Undertale AU Wiki (Accessed 4/18/2020)
  6. ^ Flowerfell, Undertale AU Wiki (Accessed 4/18/2020)