Sans/In Battle

Sans
You feel like you're going to have a bad time.
Battle Information
Music playing	MEGALOVANIA (During the main battle) The Choice (When attempting to offer mercy)
HP	1 (Scripted to die in one hit)
AT	1 (No invulnerability after being hit, damage is converted into KR)
DF	1 (Dodges every attack for 22 turns)
EXP On Kill	Auto LV up to 20
GOLD On Win	0
ACTs	Check

it's a beautiful day outside.
birds are singing, flowers are blooming...
on days like these, kids like you...
Should be burning in hell.

Sans

Sans is the final boss of the Genocide Route and his boss fight is the most difficult in the game. Similar to the Neutral and Pacifist routes, he waits for the protagonist at the Last Corridor. However, he skips LOVE judgement, apologizes to Toriel, and begins an encounter.

Sans alters the SOUL Mode to Red or Blue, and uses White and Light Blue Attack Types. He uses various complex bone patterns, straight beams from "Gaster Blasters," and telekinesis. Excluding telekinesis, Sans's attacks inflict KARMA, a mechanic that dwindles the protagonist's HP over time like a poison.

Attacks

SOUL Modes

Excluding the first attack, Sans sets the SOUL to Blue mode for the first half of the battle. At the halfway point, Sans also occasionally swaps to Red mode, or uses a mix of Red and Blue.

Patterns

Sans's attack style is focused on platforming, spontaneity, and forcing quick decisions. Unlike other battles in Undertale, Sans always attacks first at the start of the battle.

Strongest attack

There are three segments for the first attack:

1. Sans activates Blue Mode and throws the SOUL to the bottom of the Bullet Board. A red rectangle indicator appears at the bottom and after a while bones cover the area.
2. The SOUL turns Red, and a wave of bones appear from the left side of the Bullet Board. These bones cover the top and bottom of the Bullet Board and creates a somewhat linear gap between them.
3. Sets of Gaster Blasters appear to shoot beams 4 times:
   • A set of four that shoots and covers the sides of the Bullet Board.
   • Four that shoot from the corners of the Bullet Board, making the beams appear as an "X" pattern.
   • A replica of the first, which covers the sides of the Bullet Board.
   • Two large Gaster Blasters emit large beams from the left and right side of the Bullet Board. The beams cover nearly the entire board except the top and the bottom.

1st through 12th attack turns

Blue SOUL mode is active for all of these attack turns.

1. Bone lines: 16 lines of bones, which requires 8 small jumps to get through the gaps. The lines of bones appear from the left and right sides of the Bullet Board, and has a small gap between them. These are made out of small bones from the bottom, and longer bones from the top of the Bullet Board.
   • The harder variant has the bone lines appearing quickly.
2. Blue bones: 6 set of bones appear from the left side of the Bullet Board, then another 6 appear from the right, for a total of 12 bones. The left side is a pattern of long light blue bone, then a short bone. The right is a pattern of a short bone, then long light blue bone.
3. Randomized bone lines: Randomized pairs of bone lines appear from the left and right. The bone lines can appear at varying speeds. These force the protagonist to jump at varying heights through the gaps. It can range from a total of 3 through 6 jumps required.
4. Platforms 1: Starting from the left is three green rectangle platforms atop a sea of bones. Near the top of the third platform is a set of three bones, and at the end of the sea of bones is a long bone from the top.
5. Platforms 2: Starting from the right is a longer platforming segment with seven green platforms. The fourth, fifth, and sixth platforms have a bone above each platform. The last platform has long bones appearing from the top left and bottom right sides.
6. Platforms 3: A segment with moving, long green platforms and bones that can block a path. The bottom platforms appear from the right, and the higher platforms appear from the left.
7. Platforms 4: The SOUL appears on a small green platform that moves left and right. The bottom of the Bullet Board is covered by a sea of bones, and three sets of levitating bones cover the near center. The center line of bones descends, while the outer two lines rises.
   • The harder variant has more bones for the outer lines of rising bones.
8. Horizontal bone slides: Sets of small bones appear on the bottom, from the left, and large bones appear at the top, from the right.
9. Platforms with Gaster Blasters: A segment with small moving green platforms and has thin Gaster Blasters. The bottom platforms appear from the right, and the top appear from the left. Thin Gaster Blasters appear to shoot above or below platforms from the left or right sides.

If other options were used, Sans rotates between attacks until he offers false mercy. It is harder versions of Bone lines and Platforms 4, Randomized Bone lines, Horizontal bone slides, and Platforms with Gaster Blasters.

False mercy

• Sans pauses the battle and attempts to offer mercy. If accepted, he covers the entire Bullet Board with bones until the protagonist's HP is 0. Shortly after, the exclusive, sped up and high-pitched Dogsong Game Over screen appears. Sans taunts the protagonist for their defeat, then in a serious manner, he tells protagonist that if they are really friends, protagonist wouldnt come back and just to reset. As he would not actually spare them at this point.

13th through 23rd attack turns

After getting past his mercy offer, the fight then includes Sans constantly flickering the screen as he transitions through his attacks, uses his "Gaster Blasters" more often, and even attacks the protagonist as they navigate the menu (though menu attacks only deplete their HP to 1 and cannot kill them). After enough turns, Sans then performs an attack lasting roughly 40 seconds, launching some attacks shown previously at the protagonist, including a large array of bones, a bevy of "Gaster Blasters", and many slams against the Bullet Board's walls (the protagonist's HP is depleted by 1 for every slam but stops once their HP reaches 1, preventing the attack from killing them).

"Special attack"

Once the protagonist survives his final attack, Sans uses his "special attack," where he stalls indefinitely, preventing the protagonist from accessing the menu, in hopes for them to withdraw out of boredom. After Sans finishes his dialogue, the protagonist must wait as his eyes gradually close until he eventually falls asleep, then slowly push the Bullet Board to the left and then downwards to access the "FIGHT" option while Sans does not notice them. The Bullet Board cannot be pushed elsewhere, and should the protagonist try to push to the left before Sans falls asleep, his eyes fully open up and his left eye flashes light blue and yellow (the colors for patience and justice) as he teleports the protagonist's SOUL back to the middle of the Bullet Board. This resets the time it takes for him to fall asleep.

Sans defeated

When the protagonist's SOUL hits the FIGHT option, they attack Sans. He wakes up and dodges protagonist's slash, only to be defeated by another attack that does 9999999 damage. Sans does not perish immediately, but starts to leak a red fluid of some sort. He warns the protagonist one last time and then Sans "walks into grillby's," (thats what he says), a dust sound effect appears, ending the fight, raising the protagonist's LOVE to 20 and HP to 99.

Mechanics

Unlike other bosses, Sans dodges up to 24 attacks, resulting in a 'miss', thus negating his need for high defense or health.

Through KR, Sans drains the protagonist's HP by 1 per frame, and can inflict multiple weak hits (up to 30) at a rapid rate. The amount of KR inflicted varies per attack type, and its duration is based on how much the SOUL was hit. This dwindling effect is also active during the protagonist's turn. However, the protagonist cannot be killed until KR's effect ends. The exception is if the SOUL gets damaged at 1 hp during one of Sans's KR inflicting attacks. Notably, the protagonist's INV can slow down the effects of KARMA.

Some attacks involve rectangle platforms, which is green on top, and is white on the bottom. These can be seen outside of the Bullet Board. The SOUL can jump through the white portions of these platforms and rest on top of one.

After Sans pretends to spare the protagonist and fails, he also attacks with bones on the action menu. These bones deal 1 damage per hit, does not inflict KR, and cannot kill the protagonist.

Strategy

To progress the fight, the protagonist must keep attacking Sans, as checking reveals that he cannot dodge forever; performing any other action besides FIGHT, including skipping/missing the attacks, does not progress the battle, as his dialogue does not appear or advance. Sans cannot be spared, as the only way to end the fight is for the protagonist to defeat him.

While most equipment is useless as DF does not affect Sans's constant 1 damage and AT does not affect his dodging, nor does it help the fact that he only has 1 HP and Defense, there are several exceptions:

• The Burnt Pan's boost to healing items can be very useful for the protagonist to recover health.
• If the protagonist has INV-boosting items equipped, such as the Cloudy Glasses and the Torn Notebook, damage is ignored every few frames, and KR drains slower.
  • Note: While the Stained Apron and Temmie Armor can heal 1 HP every other turn and the latter also boosts some INV, they are both unobtainable in the Genocide Route.

Sans does not attack the protagonist when he attempts to offer false mercy, which allows the protagonist to heal as many times as they need to.

Quotes

Pre-Battle

• heya.
• you've been busy, huh?
• ...
• so, i've got a question for ya.
• do you think even the worst person can change...?
• that everybody 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.
• [Refight]
  • 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.
• let's just get to the point. [Refight #12+]
• [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?

Battle

• del [Unused]
• [First time and only until the opening attack is survived once]
  • it's a beautiful day outside.
  • birds are singing, flowers are blooming...
  • on days like these, kids like you...
  • Should be burning in hell. [Sans's voice is not present for this line.]
  • huh.
  • always wondered why people never use their strongest attack first.
• [After surviving the opening attack once; abruptly cuts to the opening attack]
  • it's a beautiful day outside.
  • birds are singing,
  • anyway, as i was saying, it's a nice day out.
  • why not relax and take a load off?
• [After surviving the opening attack 2+ times]
  • ready?
  • here we go.
• [Attack #1] what? you think i'm just gonna stand there and take it?
• [Attack #2]
  • our reports showed a massive anomaly in the timespace continuum.
  • timelines jumping left and right, stopping and starting...
• [Attack 3] until suddenly, everything ends.
• [Attack #4]
  • heh heh heh...
  • that's your fault, isn't it?
• [Attack #5] you can't understand how this feels.
• [Attack #6]
  • knowing that one day, without any warning...
  • it's all going to be reset.
• [Attack #7] Look. i gave up trying to go back a long time ago.
• [Attack #8] And getting to the surface doesn't really appeal anymore, either.
• [Attack #9]
  • cause even if we do...
  • we'll just end up right back here, without any memory of it, right?
• [Attack #10]
  • to be blunt...
  • it makes it kind of hard to give it my all.
• [Attack #11]
  • ... or is that just a poor excuse for being lazy...?
  • hell if i know.
• [Attack #12]
  • all i know is... seeing what comes next...
  • i can't afford not to care anymore.
• [Attack #13]
  • ugh... that being said...
  • you, uh, really like swinging that thing around, huh?
  • ...
  • listen.
  • i know you didn't answer me before, but... [Normal spare dialogue]
    • 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 this, ok?
    • just lay down your weapon, and...
    • well, my job will be a lot easier.
  • friendship... [Spare dialogue if the protagonist has previously tried sparing Sans]
    • it's really great, right?
    • let's quit fighting.
    • [Spare]
      • ...
      • 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.
      • [Fight screen after getting betrayed by Sans] geeettttttt dunked on!!!
        • if we're really friends...
        • you won't come back.
• [Attack #14]
  • welp, it was worth a shot.
  • guess you like doing things the hard way, huh?
  • [if the protagonist previously tried sparing Sans]
    • 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, ok?
• [Attack #15]
  • 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 #16]
  • and maybe all they needed was... i dunno.
  • some good food, some bad laughs, some nice friends.
• [Attack #17]
  • but that's ridiculous, right?
  • yeah, you're the type of person who won't EVER be happy.
• [Attack #18]
  • you'll keep consuming timelines over and over, until...
  • well.
  • hey.
  • take it from me, kid.
  • someday...
  • you gotta learn when to QUIT.
• [Attack #19] and that day's TODAY.
• [Attack #20]
  • cause... y'see..
  • all this fighting is really tiring me out.
• [Attack #21]
  • and if you keep pushing me...
  • then i'll be forced to use my special attack.
• [Attack #22]
  • 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 wanna see it, now would be a good time to die.
• [Attack #23]
  • well, here goes nothing...
  • are you ready?
  • survive THIS, and i'll show you my special attack!
• [After final attack]
  • 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 gonna 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.
• [Attack #24] Heh, didja really think you would be able to-
• [Attack #25 (automatic)]
  • ...
  • ...
  • ...
  • 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

Any phase

• [Check] The easiest enemy. Can only deal 1 damage.
  • [Check after Attacking atleast once] Can't keep dodging forever. Keep attacking.
• [Neutral, 0-10 KR] You felt your sins crawling on your back.
• [Neutral, 10-20 KR] You felt your sins weighing on your neck.
• [Neutral, 20-30 KR] KARMA coursing through your veins.
• [Neutral, 30-40 KR, though difficult to trigger due to the rate at which KR drains at this stage] Doomed to death of KARMA!
• Skipped ahead. [Neutral; unused]

1st phase

• You feel like you're going to have a bad time. [Encounter]
• Just keep attacking. [Neutral]
• Sans's movements grow a little wearier. [After attacking four times]
• Sans's movements seem to be slower. [After attacking nine times]
• Sans is sparing you. [When Sans pretends to spare the protagonist]

2nd phase

• Felt like a turning point. [Neutral]
• The REAL battle finally begins. [Neutral]
• Reading this doesn't seem like the best use of time. [Neutral]
• Sans is starting to look really tired. [Neutral]
• 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]

Trivia

• During the battle with Sans, Serious Mode is activated.
• 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.
• Sans's stats show that he has 1 attack, 1 defense, 1 health and is an easy enemy, although the battle itself shows the opposite with him.
  • Even Monster Kid's stats are slightly higher.
• Prior to version 1.05 of Undertale, several lines of Sans's pre-fight dialogue cut off early due to a glitch.
• Sans's Check text is partially deceptive, since his 1-damage attacks are only threatening with his ability to bypass invincibility frames.
• If the game is closed during the battle, Sans counts this as a death when making his next comment before battling the protagonist again.

The Gaster Blaster's firing animation.

• Sans and Papyrus's bones have different sprites; Sans has more rounded attacks, while Papyrus's are flatter on the top and bottom.
• The sound of Sans's Gaster Blasters consists of two sounds used by Asriel. First, the text sound when "ASRIEL DREEMURR" displays at the beginning of his battle (mus_sfx_a_gigatalk), and second, a beam sound used by Asriel's Chaos Buster and final attack (mus_sfx_rainbowbeam_1).
• When blue mode is active, choosing an option (FIGHT, ACT, ITEM, or MERCY) may cause the SOUL to fall; this can be predicted if the SOUL is in the lower part of the button.
• Surviving Sans's Fake Mercy offer will cause the bones to go down and disappear and Sans continues shrugging while the game softlocking
• Sans is one of only four characters who interfere with the protagonist's battle options, along with Asgore, Omega Flowey (also as known as photoshop flowey) and Asriel; his bone attacks on the menu during the protagonist's turn add urgency that is absent from most other fights in the game.
• As with Mad Dummy's battle outside of Genocide, Sans can trap the protagonist by simply refusing to end his turn. However, the protagonist is able to break his influence, whereas Mad Dummy stays until driven away by Napstablook.
• The last two attacks on Sans, including the one that connects, always use the animation from the Real Knife, regardless of the weapon that is currently equipped. The same goes for the automatic attacks on Asgore, Flowey and Omega Flowey.
• When Sans is hit, the wound on his body contains visible red liquid, unlike any other monster (as monsters are made primarily of magic). It is unclear whether this means Sans is a special type of monster, not a monster at all.
• Sans is the only monster to not have the health bar visible at any time, as he dies in one successful hit.
• "Song That Might Play When You Fight Sans," which is located between "Undertale" and "The Choice" in the Undertale Soundtrack, may have been added to misguide players into believing that Sans would have a battle at some point in the Neutral Route.