Sans is the final boss of the Genocide Route. His fight is widely considered the most difficult in the game among players. Sans uses bones and straight beam projectiles that emerge from skeletal faces, known internally as "Gaster Blasters," in his attacks.


Soul Modes

Sans starts by using red mode, but frequently switches the protagonist's SOUL between red and blue mode.


Sans often utilizes gravity to make his attacks require platforming skills, such as slamming the protagonist's SOUL against the walls of the bullet box and making the protagonist jump through gaps between bones and onto platforms. Sans uses regular and light blue attacks.

Sans immediately starts off the battle with his "strongest attack": an array of bones and beams. He is the only enemy in the game that has the first turn rather than the protagonist, which can catch the unwary off-guard.

Eventually, Sans will offer mercy; if accepted, he covers the entire Bullet Board with bones until the protagonist's HP is drained to 0. This results in the famous Game Over screen that is only seen on this occasion. If this happens, death is inevitable, although if the player hacks the game to get unlimited health Sans will continue to shrug but the bones will disappear and the game cannot progress without save reloading or other hacks.

After that, he starts to employ randomly cobbled attacks made of "bone stabs", "Gaster Blasters", and "bone slides". He will also start to use "Gaster Blasters" more frequently and aggressively, and will start using vertical "bone slides" rather than just the horizontal variation he uses during the first half. After a while, he does his biggest attack, which is a massive army of Gaster Blasters.


To progress the fight, the protagonist must keep attacking Sans as checking reveals that he cannot dodge forever; performing any other action besides FIGHT will not progress the battle as his dialogue will not appear or advance. Also, skipping/missing the attacks will not progress the battle either.[1]

After getting past his mercy offer, the fight then includes Sans constantly flickering the screen as he transitions through his attacks, uses his beams more often, and even attacks the protagonist as they navigate the menu (though menu attacks will only deplete the HP to 1). After enough turns, Sans will then perform an attack lasting roughly 40 seconds, launching some attacks shown previously at the protagonist, including a bevy of "Gaster Blasters", a large array of bones, and many slams against the Bullet Board's walls.

Once the protagonist survives his final attack, Sans will use his "special attack," which cannot do damage to the protagonist. Sans abstains from attacking and stalls for his turn, preventing the protagonist from accessing the menu in hopes of the protagonist becoming bored as time flies.

If one tries to move to the left before Sans falls asleep, he will wake up as his eye flashes blue/yellow, and the SOUL will be teleported back to the center of the bullet board. After several minutes and dialogue, Sans will eventually fall asleep, allowing the protagonist to push the Bullet Board towards the FIGHT button.

After pressing it, Sans will wake up and dodge the first attack, only to be struck down with another automatically ambushed attack, instantly finishing Sans off for good. Unlike most enemies, however, he starts to leak red fluid of some sort, (which is theorized to be injected determination, ketchup, or even blood) then limps offscreen before dying, raising the protagonist's LOVE to 20 and HP to 99.


Sans dodges attacks (thus negating the need for high defense or health) and removes the protagonist's health slowly further through Karmic Retribution, although more INV can slow this down. The latter statement means that his attacks (true to his flavor text when Checking him) deal 1 damage per frame the protagonist's SOUL is colliding with them, allowing Sans to inflict weak hits at such a rapid rate (up to 30 times per second without extra INV) that it adds up to massive chunks of the protagonist's HP being gone in seconds. Some attacks involve platforms the protagonist's SOUL can rest upon safely. From his false plea for the protagonist to stop fighting onwards, Sans will also attack the protagonist in the action menu.



(First time)
  • heya.
  • you've been busy, huh?
  • ...
  • so, i've got a question for ya.
  • do you think even the worst person can change...?
  • that everyone can be a good person, if they just try?
  • heh heh heh heh...
  • all right.
  • well, here's a better question.
  • do you wanna have a bad time?
  • cause if you take another step forward...
  • you are REALLY not going to like what happens next.
  • welp.
  • sorry, old lady.
  • this is why i never make promises.
  • heya.
  • you look frustrated about something.
  • guess i'm pretty good at my job, huh?
(Refight 2)
  • hmm. that expression...
  • that's the expression of someone who's died twice in a row.
  • suffice to say, you look really... unsatisfied.
  • all right.
  • how 'bout we make it a third?
(Refight 3)
  • hmm. that expression...
  • that's the expression of someone who's died thrice in a row.
  • ...
  • hey, what comes after "thrice," anyway?
  • wanna help me find out?
(Refight 4)
  • hmm. that expression...
  • that's the expression of someone who's died quice in a row.
  • quice? frice?
  • welp, won't have to use it again anyways.
(Refight 5)
  • hmm. that expression...
  • that's the expression of someone who's died five times in a row.
  • convenient, huh? that's one for each finger.
  • but soon...
  • you'll need a cool mutant hand to count all of your deaths.
(Refight 6)
  • hmm. that expression...
  • that's the expression of someone who's died six times in a row.
  • that's the number of fingers on a mutant hand.
  • but soon...
  • you'll need to find a mutant hand with even more fingers.
(Refight 7)
  • hmm. that expression...
  • that's the expression of someone who's died seven times in a row.
  • hey, that's good. seven's supposed to be a lucky number.
  • who knows, maybe you'll hit the jackpot...
  • and that number will multiply tenfold.
(Refight 8)
  • hmm. that expression...
  • that's the expression of someone who's died eight times in a row.
  • that's the number of fingers on a spider.
  • but soon...
  • wait, don’t spiders have legs?
(Refight 9)
  • hmm. that expression...
  • that's the expression of someone who's died seven times in a row.
  • nope, wait, that's definitely nine, sorry.
  • or was it ten?
(Refight 10)
  • hmm. that expression...
  • that's the expression of someone who's died ten times in a row.
  • hey, congrats! the big one-oh!
  • let's invite all your friends over for a big shindig.
  • we can have pie, and hot dogs, and...
  • hmmm... wait. something's not right.
  • you don't have any friends.
(Refight 11)
  • hmm. that expression...
  • that's the expression of someone who's died eleven times in a row.
  • well, give or take.
  • there's nuance to this stuff.
  • don't think i'll be able to count very well from here.
  • count for me, ok?
  • we'll start from 12.
(Refight 12 and beyond)
  • let's just get to the point.
(Refight after killing Sans)
  • ...
  • that expression that you're wearing...
  • ...
  • well, i won't grace it with a description.
(Refight after killing Sans two or more times)
  • ...
  • that expression that you're wearing...
  • ...
  • you're really kind of a freak, huh?

  • it's a beautiful day outside. birds are singing, flowers are blooming... on days like these, kids like you...
  • Should be burning in hell. [Only until the player has survived the opening attack. Sans's voice isn't present when this line is said.]
    • huh. always wondered why people never use their strongest attack first. [First time the player survives the opening attack]
  • anyway, as i was saying, it's a nice day out. why not relax and take a load off? [After surviving the opening attack twice]
  • ready? [After the opening attack has been survived twice]
  • here we go. [After surviving the opening attack 3+ times]
  • what? you think i'm just gonna stand there and take it? [Attack]
  • our reports showed a massive anomaly in the timespace continuum. timelines jumping left and right, stopping and starting... [Attack #2]
  • until suddenly, everything ends. [Attack #3]
  • heh heh heh... that's your fault isn't it? [Attack #4]
  • you can't understand how this feels. [Attack #5]
  • knowing that one day, without any warning... it's all going to be reset. [Attack #6]
  • look. i gave up trying to go back a long time ago. [Attack #7]
  • and getting to the surface doesn't really appeal anymore, either. [Attack #8]
  • cause even if we do... we'll just end up right back here, without any memory of it, right? [Attack #9]
  • to be blunt... it makes it kind of hard to give it my all. [Attack #10]
  • ... or is that just a poor excuse for being lazy...? hell if i know. [Attack #11]
  • all i know is... seeing what comes next... i can't afford not to care anymore. [Attack #12]
  • ugh... that being said... you, uh, really like swinging that thing around, huh? ... listen. [Attack #13]
    • i know you didn't answer me before, but... somewhere in there. i can feel it. there's a glimmer of a good person inside of you. the memory of someone who once wanted to do the right thing. someone who, in another time, might have even been... a friend? c'mon, buddy. do you remember me? please, if you're listening... let's forget all of this, ok? just lay down your weapon, and... well, my job will be a lot easier. [Normal spare dialogue]
    • friendship... it's really great, right? let's quit fighting. [Spare dialogue if the player has previously tried sparing Sans]
  • ... you're sparing me? finally. buddy. pal. i know how hard it must be... to make that choice. to go back on everything you've worked up to. i want you to know... i won't let it go to waste. ... c'mere, pal. [Spare]
    • geeettttttt dunked on!!! if we're really friends... you won't come back. [Game over screen after sparing Sans]
  • welp, it was worth a shot. guess you like doing things the hard way, huh? [Attack #14]
    • woah, you look REALLY pissed off... heheheh... did i getcha? well, if you came back anyway... i guess that means we never really WERE friends, huh? heh. don't tell that to the other sans-es, okay? [Attack #14 if the player previously tried sparing Sans]
  • sounds strange, but before all this i was secretly hoping we could be friends. i always thought the anomaly was doing this cause they were unhappy. and when they got what they wanted, they would stop all this. [Attack #15]
  • and maybe all they needed was... i dunno. some good food, some bad laughs, some nice friends. [Attack #16]
  • but that's ridiculous, right? yeah, you're the type of person who won't EVER be happy. [Attack #17]
  • you'll keep consuming timelines over and over, until... well. hey. take it from me, kid. someday... you gotta learn when to QUIT. [Attack #18]
  • and that day's TODAY. [Attack #19]
  • cause... y'see... all this fighting is really tiring me out. [Attack #20]
  • and if you keep pushing me... then i'll be forced to use my special attack. [Attack #21]
  • yeah, my special attack. sound familiar? well, get ready. cause after the next move, i'm going to use it. so, if you don't want to see it, now would be a good time to die. [Attack #22]
  • well, here goes nothing... are you ready? survive THIS and i'll show you my special attack! [Attack #23]
  • huff... puff... all right. that's it. it's time for my special attack. are you ready? here goes nothing.
  • yep. that's right. it's literally nothing. and it's not going to be anything, either. heh heh heh... ya get it? i know i can't beat you. one of your turns... you're just gonna kill me. so, uh. i've decided... it's not gonna BE your turn. ever. i'm just gonna keep having MY turn until you give up. even if it means we have to stand here until the end of time. capiche?
  • you'll get bored here. if you haven't gotten bored already, i mean. and then, you'll finally quit.
  • i know your type. you're, uh, very determined, aren't you? you'll never give up, even if there's, uh... absolutely NO benefit to persevering whatsoever. if i can make that clear. no matter what, you'll just keep going. not out of any desire for good or evil... but just because you think you can. and because you "can"... ... you "have to".
  • but now, you've reached the end. there is nothing left for you now. so, uh, in my personal opinion... the most "determined" thing you can do here? is to, uh, completely give up. and... (yawn) do literally anything else.
  • heh, didja really think you would be able- [Attack #24]
  • ... ... ... so... guess that's it, huh? ... just... don't say i didn't warn you. welp. i'm going to grillby's. papyrus, do you want anything?
Flavor Text
  • The easiest enemy. Can only deal 1 damage. [Check]
    • Can't keep dodging forever. Keep attacking. [If attacked at least once]
  • Just keep attacking [After attacking once]
  • Sans's movements grow a little wearier. [After attacking four times]
  • Sans's movements seem to be slower. [After attacking nine times]
  • You feel like you're going to have a bad time. [Encounter]
  • You felt your sins crawling on your back. [Neutral, 0-10 KR][2]
  • You felt your sins weighing on your neck. [Neutral, 10-20 KR][2]
  • KARMA coursing through your veins. [Neutral, 20-30 KR][2]
  • Doomed to death of KARMA! [Neutral, 30-40 KR, though unused due to programming errors][2]
  • Sans is sparing you. [When Sans pretends to spare the protagonist]
  • The REAL battle finally begins. [Neutral, 2nd phase]
  • Reading this doesn't seem like the best use of your time. [Neutral, 2nd phase]
  • Felt like a turning point. [Neutral, 2nd phase]
  • Sans is starting to look really tired. [Neutral, 2nd phase]
  • Sans is preparing something. [Neutral, when he talks about his special attack]
  • Sans is getting ready to use his special attack. [Neutral, when he talks about his special attack again]



The Gaster Blaster's firing animation.

  • The "Gaster Blaster" animation features the skeletal attack's left and right eyes flickering, similar to how Sans has one pupil while he performs psychokinesis.
  • Sans is the only character that dies off-screen, which leads players to question if he actually died or if he fled from battle. However, it is likely the case he died, as the death sound plays and your EXP and LOVE increase.
  • Sans's initial dialogue in his fight parallels Asgore's dialogue in a Neutral Route. They both open with "Birds are singing, flowers are blooming..." The following dialogue differs between the characters.
  • In battle he says, "... you'll just keep going. not out of any desire for good or evil... but just because you think you can..." This quote could refer to the "completionist" gamers who do everything possible in a game because they feel they must.
  • After hacking in infinite health and surviving Sans's Mercy attack, the bones will fall, and Sans will remain shrugging. One cannot do anything other than restarting the game to progress.[3]
  • Karmic Retribution drains more slowly if the player has INV-boosting items equipped, such as the Cloudy Glasses and the Torn Notebook.
  • When blue mode is active, choosing an option ([Fight],[Act],[Item] or [Spare]) may cause the soul to fall, this can be predicted if the soul is in the lower part of the button.


  1. Not attacking Sans - YouTube
  2. 2.0 2.1 2.2 2.3 Some stuff about the Sans fight that you may not have known - uselessundertalefacts, Tumblr
  3. Sans spares a hacker - YouTube

Start a Discussion Discussions about Sans/In Battle

  • Was Sans holding back?

    7 messages
    • He was giving it his all. He was also 'teleporting' you when he began switching different attack patterns several times in one turn. ...
    • ^ Yeah this, right here I thinks its possible he was holding back during the 1st phase of the fight, but not the 2nd
  • So I did a Thing... (Minor Spoiler Warning)

    20 messages
    • WDG6ster..66 wrote:my favret sans is inctale sans