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Underfell Sans

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Underfell Sans
Type Ghost
Developer(s) ChangeSide
Origin Undertale (AU)
Language(s) English
Sakura Sans
Release date August 26th, 2018

Content warning: Extremely dark jokes, topics including: sex, pedophilia, children, babies, racism, cannibalism, and macabre.

Underfell Sans is a ghost by ChangeSide, of Sans from an Underfell AU. He is a moody skeleton that makes dark/edgy jokes, and has complex systems for his mood, feelings towards the user, and various unlockable options.


  • Mood system and feelings system
  • Toggleable animations
  • Minigames, where the user can earn G
  • Shop system, where the user can spend G
  • Variable backup system
  • Music player1)
  • Customizable pronouns
  • Dark humor, which can be filtered by topic
  • Customizable alarms
  • Toggleable color options for the menu text


All minigames can be used to earn G. The user can bet various amounts of G in Flip A Coin and Rock Paper Scissors. Winning the game will earn the user twice the G they bet, while losing will make them lose the amount of G they bet.

  • Flip A Coin
    • The user can pick heads or tails, then flip a coin to see if they won
  • Rock Paper Scissors
    • The user can pick from rock, paper, or scissors, then click Start to compare their throw to Sans's
  • Tic Tac Toe
    • The user and Sans can take turns picking spaces, with the goal of filling in 3 spaces in a row with their own symbol
  • Gold Digger
    • The user is presented with a field of boxes, and can click each one to see what's underneath. Some boxes give G, while others take G


The user can earn G to spend through minigames, and will also passively earn G slowly just by keeping Sans open.

The Sans' Shop tab contains an option to pay off Sans's tab, which will take the user to the developer's Ko-Fi and Patreon information. Ko-Fi and Patreon supporters are given special content and shoutouts in the ghost. This tab also has an option to buy snow.

The Undernet-buy tab has multiple items that the user can buy, which may be used at various times if they are in the inventory. This includes a red rose, painkillers, spicy mustard, and gross food.

Shell overhaul

Underfell Sans's shell was overhauled as a part of the 2.0 update, replacing the old one entirely. The old version is shown below.


Underfell Sans comes bundled with a custom balloon called underfell_cs, an Undertale themed balloon that uses the Comic Sans font.

Click to reveal balloon images.

The music player is currently bugged and many functions do not work.