Asgore Dreemurr.png
* Golly, this sure is terrible

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After W.D. Gaster injected him with a small dose of condensed determination, he went absolutely insane. He went on a spree, killing his family and friends.

Since he killed everyone in the underground, he also killed his brother Papyrus... but not even that, he even ripped his skull out of his torso, breaking and removing his lower jaw.

Sans does this to every formidable foe he fights. Instead of turning them to dust, he decapitates them, skewers their eternally suffering head on a bone and keeps them as trophies. He carries papyrus’ head around monsters to show that he won't hesitate to kill them, as he tortured his own brother.



His appearance is similar to a regular Sans, clothing wise. His clothes are more tattered and has small bits of dust and his skull has cracks on his top part because that's where his victims usually attack. He has unusually small eyes to show his insanity. His hands, clothes and teeth are covered in bloodstains, and he has an unnaturally wide smile. In his left hand, he holds Papyrus' decapitated skull (which has bone impaled through it, missing a jaw, and is bloody).


He is a crazy psychotic killer with no mercy. He's also smiling widely and twitching, which truly shows his insanity.

Canon Insanity Sans:

The creator, Zero Danteero, made a 2 part special on how he made Insanity sans. Here is the full transcript of the story that was told in part 2:

Sans was infected by one of his fathers’ experiments, infecting his mind and causing him to become a psycho. The chemical removed crucial parts of his brain; happiness, confusion, common sense and empathy. When meeting with chara he realized the power of reset. The ability to commit murder and get away with it. He uses this power to reset and kill everyone. He enjoys every moment of it.

Not only that, but he has a sick personality and nobody can and nobody can stop him with how powerful he is.

When you fall down (story):

Genocide will begin, but chara will realize that everyone is missing. Everything is empty. Chara will occasionally see sans in a distance throughout exploring. Until the final halls, where finally chara feels a presence. Sans, standing at the end of the hall, papyrus skewered and held in his left hand. Sans begins to twitch and suddenly launches at chara. He drops papyrus’ head and starts throwing attacks at chara. Sans doesn’t only use bones, blasters and gravity switch. But he will wield bones in an attempt to stab chara and impale them. He will also punch chara, killing them every time with his power.

And it's an endless loop of limitless power vs an immortal foe. Chara will sit in the save menu for hours thinking of a plan to kill him. Maybe they’ll eventually find a way.


insanity sans with a blaster (made by UTF/The Undertale and BFDI Fan) Zeero Danteero approved of this himself.

Sharp bones

Unlike other Sanses, his bones are Sharp and can deal big damage. he holds them in his hands and stabs you with them. these vary in size

Hard hitting punches

he uses this and the bones more than his blasters and telekinesis, as he prefers hand-to-hand combat.

3005e6dd5223e04e9e5fe456ccdb4d99ea775a9er1-480-360v2 00.jpg

Insanity blaster

Like a normal gaster blaster, except it is cracked and blooded. he doesn't use this as much as its a bore for him.


its the same as normal, but he also doesn’t use it that much. he prefers hand-to-hand combat =)


Fanon Insanity!Sans:

Has the normal state, but it also has a RED form which is much more powerful (you can call this his "Hard mode" or his "Mirrored Insanity Phase 3 Appearance")

Short info on his Hard-mode appearance:

  • It's pretty much the same otherwise, except that he has a big red slash/strike in his chest indicating that he was struck by Dust Sans but he used the Determination which he got from killing Dust Sans to refuse death.
  • There is blood or most likely Determination flowing downward out of his eyes and has the same red substance flowing out of the top of his head.
  • He has glowing red eyes with a white color on the inside.
  • He is still holding Papyrus' head except that the top part of it is more cracked than before
  • See all the Hard-mode sprite here:

Update on the Hard-mode!

ReveX just posted a video, but this time, it's a v2 on Insanity's Hard-mode! He posted this on October 6, 2020. If you haven't seen the video yet, here's the link:

Anyway, here's the appearance info on his v2 Hard-mode:

Pretty much the same, with some noticeable differences.

  • He no longer has that huge slash on his torso area
  • He no longer holds Papyrus' head
  • Streaks of blood/determination no longer flow from the top of his head and eye sockets.
  • His left eye socket is raised one pixel higher than his right
  • His skull and torso are shaking at EXTREME speeds, and when I mean extreme, I mean EXTREME.

All changes not mentioned remains the same.


W. D. Gaster

Gaster created an injection of condensed Determination, and used it on Sans to see if a monster could access the power of reset. He knew there was a possibility of Sans becoming insane, but didn't think it would actually happen.


Sans brutally kills him and skewers his head on a bone (like everyone else's). But he keeps Pap’s head as a trophy and a reminder of his brother.


Insanity sees them as just another thing to kill.


Insanity hates them more than anything.


Mirrored Insanity Enemy/Insanity Duo Teammate/Murder Time Trio Phase 2 Teammate.

Killer + Dust!Sans

Insanity helps them in Phase 2 of Murder Time Trio, it is unknown what his relationship/opinion of them is.

Mirrored Insanity

Dust!Sans was roaming around his universe, the same as Insanity! Sans when all of a sudden, Insanity!Sans was somehow in Dust's AU, causing two murderers to start a fight, resulting both to die... or not both?

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