Sans/In Battle

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

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

Patterns
Sans's method of fighting can be seen as an opposite to Photoshop Flowey's fighting style. Whereas Flowey fights the protagonist using bullet hell techniques, Sans 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 whole Bullet Board with bones until the protagonist's HP is drained to 0.

Strategy
To progress the fight, the protagonist must keep attacking Sans as 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.

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 a "really cool regular attack," launching the entirety of his arsenal at the protagonist, including a bevy of Gaster Blasters, slams against the Bullet Board's walls, amongst his other attacks.

Once the protagonist survives his final attack, Sans will use his "special attack," which is nothing. 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 "bleed" (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.

Mechanics
Sans dodges attacks (thus negating the need for high defense or health) and removes the protagonist's INV through Karmic Retribution. The latter statement means that his attacks (true to his flavor text when Checking him) deal 1 damage per every frame the protagonist's SOUL is colliding with them, allowing Sans to inflict weak hits at such a rapid rate 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.

Quotes

 * Flavor Text
 * The easiest enemy. Can only deal 1 damage. [Check]
 * Can't keep dodging forever. Keep attacking. [If attacked at least once]
 * You feel like you're going to have a bad time. [Encounter]
 * You felt your sins crawling on your back. [Neutral, 0-10 KR]
 * You felt your sins weighing on your neck. [Neutral, 10-20 KR]
 * KARMA coursing through your veins. [Neutral, 20-30 KR]
 * Doomed to death of KARMA! [Neutral, 30-40 KR]
 * 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]
 * 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]

Trivia

 * 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.
 * 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.
 * Karmic Retribution drains more slowly if the player has INV-boosting items equipped, such as the Cloudy Glasses and the Torn Notebook.
 * The protagonist's health draining from the karmic retribution is taken from a similar mechanic in EarthBound.

Sans/Im Kampf Sans/En Combat Sans/W walce Санс/В бою