Sans/In Battle

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.

Although most enemies attack the player by using as many attacks as possible against them, Sans uses the opposite style, using minimal attacks and different soul modes to make the battle much harder.

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

Patterns
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. Intro: This is the first attack. The player is turned into the blue soul mode and must jump up to avoid bones. Then they are turned into the red soul mode and must move through a wave of bones. Then the player must move multiple times to avoid Gaster Blasters.

Bonegap: The player is turned into the blue soul mode and is presented with bones with gaps in them. They must jump up to go through the gap.

Bluebone: The player is turned into the blue soul mode. Then, they are confronted with a blue bone which height is higher than the player can jump, and a white bone that is a little higher than the SOUL. This repeats six times, albeit at a different direction.

Platform attack:  The player is turned into the blue soul mode and must jump from platform to platform to avoid the bones.

Bone slides: The player is confronted with bones at different heights. When put together they create a sort of grid of bones. The player must jump to prevent from being forced against the walls of the Bullet Board.

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. The player must FIGHT to continue.

After that, he starts to employ randomly cobbled attacks made of reflex bones, bone slides, and Gaster Blasters. He will also start to use Gaster Blasters more frequently and aggressively. After a while he uses his final attack.

Reflex bones: The player is slammed against any of the four walls of the Bullet Board. A red rectangle appears outlining the attack, and the player must jump to avoid it. This repeats multiple times.

Counterclockwise bones: The player is confronted with bones that they must move in counterclockwise circles or left and right to avoid.

Final attack: This starts with reflex bones, then moves on to counterclockwise bones. Then they must move through a wave of bones much like the first attack. After that there are more reflex bones, then a massive army of Gaster Blasters that the player must stay in the near center to avoid (unless they have Sea Tea).

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 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 to transition through his attacks, (known by some fans as black flashlights) 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.

Mechanics
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.

Quotes

 * 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]
 * 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]
 * 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]

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. 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.
 * 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.

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