UNDERFELL by ManiaKnight is a fangame being developed for an interpretation of the popular aesthetic AU, Underfell.

There is currently a demo release available for the public, while the full game is still in-development by ManiaKnight and his team.

About the AU

UNDERFELL takes place directly after Frisk completes a pacifist run from the original UNDERTALE. Everything suddenly changes as Frisk is helping the monsters move above ground to live among the humans and it’s up to Flowey, Sans & Papyrus to get Frisk back to their original reality, if they can.

Notable Changes

General Changes

  • Once the player finishes the game, they can embark on World Quests with either Sans or Papyrus and collect Upgrade Tokens for their weapons and armor.
    • All weapons and armor will automatically scale to the highest attack/highest defense value weapon/armor in the game once the main story-line has been completed. After this, Frisk can upgrade their weapon’s attack and armor’s defense further with upgrade tokens.
    • Upgrading weapons and armor will allow to tackle harder echoes and difficult bosses which would otherwise be impossible without improved equipment.
  • An “Echo” is a concept that is being implemented as UNDERFELL’s endgame. When the player initially beats UNDERFELL, they will be prompted to undertake world quests to stop a mysterious entity that has appeared in the world of UNDERFELL, in order to upgrade their gear.
    • After Frisk's gear is at an acceptable upgrade level, they will be approached by Ink!Sans, since they are the true, original Frisk. Ink!Sans will explain that the entity known as Error is destroying alternate universes, and he must be stopped before he brings about complete and utter annihilation. Once this is out of the way Frisk will be ready to embark.
    • Echoes will bring Frisk to different alternate universes, such as Nextale, Underswap, and Naturetale, this list will be expanded as time goes on. Each Echo will have around 30 minutes to an hour of playability depending on its complexity with varying difficulty levels. Once Error is stopped in that Echo, he will move to another, and so on.
      • Once said Echo is beaten that universe’s Sans (if applicable) & Papyrus (if applicable) or other character (if applicable) will follow Frisk back to UNDERFELL’s Underground, and Frisk will be allowed to recruit them to undertake world quests - they are known as Echo followers. Frisk can only have either one Sans or one Papyrus with them. Each follower will have unique dialog banter and attacks.
      • Some may react negatively and refuse to help Frisk with world quests if they’re an extremely violent individual. Others may refuse to help if they’re not a genocidal maniac.

Character Changes


  • Frisk is given a voice and personality based upon the player's actions in the game.
  • Flowey appears to be the same twisted, soulless being he was in Undertale; though, he acts as a guide for Frisk as the two traverse through the Underground.


Echo Followers

Echo “followers” are companions that follow Frisk back from their Echo world into the world of UNDERFELL to assist them in stopping Error. Frisk may bring one of these ‘followers’ along to help them with world quests. Each of them have their own pros and cons, or more commonly - benefits and disadvantages.

Blueberry (Swap!Sans)

Blueberry and his brother, Stretch joins Frisk after the initial arrival of Error. Blueberry is and will remain Frisk's most loyal follower, assuming they play nicely and by all the rules. He has good defensive capabilities and colorful conversation.

Stretch (Swap!Papyrus)

Stretch is Blueberry’s (Swap!Sans) brother and will follow wherever he goes. Stretch is the naturally stronger choice, but lacks in defensive moves. He will do whatever he can to assist Frisk, regardless of their choices - as long as his brother Blueberry remains safe.

Echo Mercenaries

Echo “mercenaries” are Echo followers that do not have their own Echo world, but join your fight against Error, regardless. They are characters such as Fresh, Cross!Sans and so on. You can unlock these ‘mercenaries’ through world quests, secrets or as you progress through the Echo story-line.

Ink (Ink!Sans)

Ink is a very interesting person, bubbly in one moment and serious when the situation demands it. He’ll stick with Frisk to the end in stopping Error, as long as it ends peacefully for everyone. Through the use of Ink’s Doodle Sphere, you’ll be able to see the wonder of alternate universes, also known as Echoes.

Cross (Cross!Sans)

Cross is dangerous, but he will prove to be a valuable ally. While he may have the same interest in stopping Error, it’s best not to mistake his actions for good intentions.