Multiverse Headcanons

When Sans accumulates so much DETERMINATION that he begins to melt, he uses the DT Extractor to siphon off just enough to keep himself from Falling Down.

Sans has a permanent grin on his face. Unlike Before, it’s not just a mask; the makeshift craniotomy Undyne performed on him made it so Sans can’t control most of his face. He expresses emotion through his upper facial structure (like frowning, etc.), but he’s had to adapt to exaggerating body language to get his point across to those who can’t read him like Papyrus can.

On the extremely rare occasion that Error falls ill, he lags, like a particularly janky video game. His memory of that time is usually corrupted, too, so when he feels like he’s coming down with something, he cocoons himself in his strings in an attempt to recuperate without the likes of Ink or Fresh trying to mess with him.

The moment Sans' skull was caved in by Undyne, the yellows in his eye turned purple as his sense of Justice was discarded for the Perseverance needed to survive.

Killer and Dust butt heads often, even after they learn to look past the differences that split their timelines. They're so similar that it frustrates them, because they fought so hard after their Choices, knowing they couldn't go back to being Sans, only to be stuck with one another. However, they bonded over one of their shared traits: WingDings. Cross could understand the font spoken aloud, but Classic Sans never could, and so neither could his Alternate Timelines; Nightmare never had a reason to know it beyond its existence; and Horror could no longer grasp the language thanks to his butchered craniotomy. So Killer and Dust combine their knowledge on the language to create shorthands of their own that even alternate versions of them wouldn't know, and use it to benefit them in both battle and mischief.

During a Genocide Route, Sans and Alphys concoct a plan together after Snowdin/SunnedOut's residents are escorted towards Waterfall/Lavarise; Sans will stay behind to fight the human and slow them down, while Alphys continues evacuating monsters. Sans, not able to remember RESETs, exactly, but knowing he's in a timeloop, tries to convince the human to stop, that they can turn their path around and end this with a bright future on the horizon. The human kills him and moves on. Napstaton NEO stands in the human's way at the border of Hotland/Coldsea, but their unfinished body is unable to handle the LV-boosted attacks from the humans, and they destroy him. They find Undyne waiting for them in the Lab, the meek scientist from previous (less Genocidal) runs nowhere to be seen; filled with DT that makes her features droop, she fights until she succumbs to the stronger source of Determination, but at least Alphys and the evacuees are safely locked behind the True Lab doors, and the human finishes out the Genocide Route (after multiple RESETs) with Papyrus' beheading, the Queen's instantaneous demise, and the cathartic shredding of fabric as they rip Temmie apart. Too bad they didn't account for their reflection in New Home coming to life and taking their SOUL when they refused to ERASE the timeline.

Sans should've died when Undyne cracked open his skull to pry out his eye. However, his source of magic and life is that eye. As long as his bones prevent anyone from entering the Core and gaining access to his eye, he is effectively immortal - whether he likes it or not. Due to the distance from the Core and Snowdin, Sans doesn't have full access to all his magic - which is why he can't summon Gaster Blasters -, but it's so powerful that even when they're on separate sides of the Underground, he can still use shortcuts and summon bone attacks.

Dusttale [Murder] Sans: [~ 5'4"] top of head level with Horror's shoulder. age range - anywhere from 23 to 27 years of age; syncs up with Undertale Sans' age prior to True Pacifist Timeline

Sometimes the Player likes to mix up how often they do a True Reset. A few times they took so long to trigger one that Sans ran out of monster food to eat and in a desperate bid to not starve, he made soup out of monster dust and snow to keep his magic levels high enough to continue effectively waging war against the human. (Sometimes he swears that he can feel the last emotions of the deceased monsters through consumption of their dust.)

Sometimes Nightmare will send one of his boys to collect Error or send him a message. Dust is the only one he won't send because the first and only time he did so, Dust was overwhelmed by the same Voices that haunt Error and had a breakdown. (Cross won't tell anyone that sometimes he hears whispers when he enters the Anti-Void too.)

Dusttale, Horrortale, and Killer!Sans all have a shard missing from their souls, because Geno!Sans was the original “Classic” Sans.  Geno, watching from the Save Screen, didn’t interfere with Horrortale or Killer!Sans’ timelines, which allowed them to grow into their own “universes” within the Undertale Universe, but Dusttale Sans was the first Sans he pulled into the Save Screen, and was the only Sans who agreed to try and help Geno accomplish his goals.  He eventually was driven mad by the countless genocidal RESETS he lived through and turned to murdering monsters in an attempt to combat Chara; Geno didn’t interfere with that timeline again.  (Error somehow gets along better with Dusttale Sans than the rest of Nightmare’s entourage, but neither one can pinpoint why.)

Because Nightmare can’t trust his group to be alone together outside of fights, Dust and Horror are sent back to their timelines once they’ve recovered from any injuries they might’ve received stirring up negative emotions in universes or scrapping with Ink and his posse.  Error, while not an official part of the group, uses the abandoned universe Nightmare claimed as his territory to get out of the Anti-Void when Outertale is being patrolled by the Star Sanses or their brainwashed “allies,” and in return, he snared the souls of Killer and Dust’s Charas and turns a blind eye to the existence of the anomalies that NIghtmare’s decided to collect.

The Riverperson’s full name is “WebDings Gaster,” with a font (language) to match, rather than WingDings like their classic!verse alters.

Nightmare Sans - Conceit [Blue] and Corruption [Yellow] Dream Sans - Justice [Yellow] / Retribution [Yellow] and Bravery [Orange]

Sans’ skull injury paralyzed his left eye socket, so now he can’t close it.  When he blinks (or really does anything that would require a functioning socket to perform, like half-lidding the eye or anything), his eye light performs the motion within his socket.

Like the rest of monsterkind, throwing away the diet of human flesh in favor for actual food was an easy choice for Sans.  However, the instincts that’d developed along with the developed lifestyle of hunting humans for a meal were harder to avoid.  Of course Sans regretted having to consume those unfortunate enough to fall underground - everyone did -, but they did what they had to to survive.  But Sans’ already frayed mental health just got worse every time he caught himself salivating at the sight or scent of humans.  He couldn’t trust himself in a crowd, and he knew that he wasn’t the only one, because monsters were becoming well-known for isolating themselves from the humans they now lived amongst.  He was more than happy to just stay in the little home he and his brother had claimed for themselves and avoid the outside world.  But while he regretted the measures they had to go through, he knew in the back of his mind that he wouldn’t be able to resist the urge to do so again, but only if it was for the survival benefit of his brother.

The more details put into a universe, the stronger the monsters’ magic of that universe is.  This is especially true for Sans, being one of the few monsters who aren’t completely in line with the timelines (hence why they can sometimes remember bits and pieces or even the entirety of past timelines).  Undertale Sans holds the baseline for “natural” power in a universe; Underfell and Underswap Sans and Papyrus are some of the few who surpass that baseline.  The only exceptions to the rule are those made specifically to hold unnaturally strong power, like Epic!Sans, Reapertale, Ink!Sans and Error!Sans, etc.