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"The sexually RPG where everybody has to [CENSORED]!"
– Undertail

Undertail is the NSFW version of Undertale and also including many Undertale AU's. This AU is directly related with Rule 34, since it's another site with adult content.

About the AU

In this AU, all the characters have a pornified version, which can vary. The plot may or not remain the same of the original universe, depending of the author, but the events are always complemented with adult and sexual themes. Sometimes the stories may have crossovers between AU's.

Although having sexual themes, Undertail also focus on a variable amount of couples in the stories, that are made by fans. This is also an AU shared between the community, having NSFW fangames, fanarts, comics and audios.


Most of the Undertale and Undertale AU characters remain physicaly the same, but they may have the main personalities and behaviors changed, in order to look more sensual and appealing for the viewer.

  • Sans may have magic tongue and genitals
  • Papyrus may have magic tongue and genitals
  • Sans and Papyrus may also have magical tentacles in some AU's
  • Sans may be more sexually appealing
  • Mettaton may or not have artificial sexual parts
  • Napstablook may or not have genitals
  • Frisk's gender can be affected in both ways, but commonly being female
  • Frisk's age can also vary depending of the author: Being a child, a teenager or an adult
  • Frisk may be more sexually appealing
  • Undyne's sex can be affected in both ways, but commonly being female
  • Flowey may or not have genitals
  • Temmie appearence may change, having a teenage body or simply a normal body
  • Chara's gender can be affected in both ways, but commonly being female
  • Asriel's behavior may change depending of his forms. In his god-like form he can be more appealing and dominant, while in his child form he is more shy and passive
  • W.D Gaster's appearance may change in some stories, but he is commonly portrayed having a magic tongue and genitals

List of AUs containing NSFW


Contains weird stuff that fills you with discomfort!
Proceed with caution...

There's nothing here, lol. gg.
Flowey Trollface


  • The Annoying Dog/Toby Fox is the less affected character in Undertail, while Sans is the most affected.
  • The AU's most affected in the Undertail fandom are Underswap, Underlust and Underfell.
  • In the official Undertale tumblr, there is a post where Mettaton asks people to use the tag "Undertail" instead of "Undertale" to post NSFW content on the internet.

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