Basic Undertale AU Info- RandomizerTale
With each reset everyone (other than frisk/chara, gaster, and maybe flowey) are randomized as different character roles. The characters still look the same and have the same names, but they act and talk as the characters they are playing as. They still have their same basic powers (except for sans who can't maintain his knowledge of resets outside his role), they just act different.
Think of it as a randomizer mod. The same art and battle assets, just different dialogue.
Sans gets nerfed for whoever he's playing as though.
The only one who realizes is the character playing as sans. Since the sans has an unnatural ability to remember every resets, this ability is transferred to who is sans and gets multiplied as they'll remember not only the pov of every sans before, but of their own povs as other characters as well. This multiplication increases exponentially to the point that sans is basically an amalgamation of everyone's povs in the underground.
The Sanses are also proficient in all types of magic now (as the body allows of course [if in Sans bod he still tired with 1hp bro and Undyne sans is an absolute BEAST] and depending on who else has been sans before). This means that genocide fights get progressively harder and harder each time a reset happens and dialogue changes too depending on who is playing sans (pap genocides usually end pretty fast though [despite everything he thinks the human can do better]).
Eventually the randomization just so happens to land on the original configuration of monsters. Seeing his chance for it to go back to normal, Sans, with everyone's pov of the resets, teleports to frisk at the beginning of the route and confronts/begs frisk to stop the madness: pleading them to be better person (paps), to stop hurting the ones they love, to let them to finally live on the surface instead of just looking at it, to just stop the resets altogether and to let this be their final reset.
After the final reset, Sans is still sans but he has EVERY monsters pov of past resets (boss and minor monster alike). Once the resets end, he isn't everyone shoved into one body, but sans with a recollection of everyone's pov of past resets. IT IS NOT D.I.D!!!! Of course he knows that he himself is sans, but he might have random episodes where he does/says things that other characters would say/like/do (ex: pose like mettaton, make a puzzle, gush like alphys over his interests, drink tea...you get the idea) due to him being forced to play as other characters in other resets. (Other chars in the au don't have this happen to them cause they don't remember previous resets)
When he is talking about past experiences during the resets, he flips between pronouns and from first/third person (I, we, she, he, they) in order to get some of feelings across (I as Sans, we as everyone's pov of resets, and others as specific people's povs).
His episodes can be broken by a) someone telling him that's not how he is, b) physical interference (ex: slap, tripping, sleep), c) him having a dissociating moment where he's like “wait a minute...”, and d) he finishes the action the episodes requires of him.
Some episodes are harder to recognize (specially ones with no final action/goal in mind. For example: getting anxious over social interaction like alphys vs making a butterscotch pie.) Episodes don't overlap each other, so he could be his sans personality while making a pie, making different jokes based on an interest change, or just extruding mettaton confidence/self-absorption).
There are certain episodes that he doesn't mind very often and will actually complete it even after he breaks out of it, specifically the task episodes like making pie/spaghetti/tea. He's learned to really like that stuff.
Some episodes that he particularly hates are when he acts/speaks like mettaton or temmie, nervously word vomits like alphys (cause there's no filter and he can say stuff he doesn't want to be said out loud [he a secretive guy]), and when he acts like Jerry because eewwwwww.
To apologize or explain his behavior after an episode he calls it the (insert character) in me or my inner (insert character). "welp my inner toriel came out again..." "sorry, kiddo, that was just the undyne in me..."
Ones he hates or are improperly timed, he sometimes just insults the characters though. "f-ing mettaton..." "alphys I like you but whyyyyyy" "*gags* ...Jerry"
Types of episodes: social (acting or talking like someone), thoughts (interests or ideals) (he HATES this one the most as it makes him feel less like him), physical (task or long-lasting [long-lasting like dancing/ posing/working out]). (Keep in mind that social and physical episodes keep sans ideas/thoughts/interests intact, he just communicates them in a way another character would say them.) Specific social or physical episodes can be triggered with high emotions (though most of the time it's just random). Extreme anger can bring out the undyne in him, fear/nervousness alphys, adoration paps, depression nabstablook, worried for someone else toriel, and so on.
Fun facts: Sans gets headaches after an episode where he acts/thinks like someone else. The severity usually depends on how long the episode is and how out of character is for him.
Fun facts 2: Following the logic of post reset sans, other characters playing as sans will think and act like the original char they are supposed to be but with sans personality. Unless episodes happen... (EX: Toriel goat jokes, undyne fish jokes.) So it leads to different character interactions where an undyne sans is more pissed off at frisk and more prone to violence. Paps sans more encouraging to be a better person. Other chars as sans also get episodes and headaches if they act like other chars other than themselves (disregarding sans cause they are supposed to be sans.). Bruh does this get complicated...
Fun facts 3: The other char’s povs only start once frisk falls underground so sans doesn't know how toriel felt leaving ashore or pap waiting outside undyne’s house to join the royal guard. That means his episodes and knowledge of others chars reset feelings/actions during povs are strictly based upon the duration the timeframe that frisk is underground.
Fun facts.4: The sanses in all the reset always has a soft spot for paps. They know that he ain't their bro but the code just has that as a priority for the sans. (Paps as sans is like my bro loves me ;n;.) They enjoy it when paps is playing alphys in genocide routes cause at least they know he won't die that route.
Fun facts 5: In an alternate version of this au when frisk ignores sans's plea for it to be their final route, sans would most likely snap and go crazy and just straight up try to murder the kid over and over again regardless if they were going to do a pacifist or geno route until they agree to his terms or until he dies (and he'll do it for all future resets [except for maybe pap sans???]) Who knows a Dusttale (see @ask-dusttale ) might even happen but the version of sans would feel less guilty bout it cause most other char povs would only confirm that other characters would just want frisk to suffer.
Fun fact 6: In another happier alternate version of this au, a version of sans confronts frisk earlier on, making them feel bad early and just wants everyone to go back to normal. So they constantly reset until they find the normal config for the final reset. The Sanses in this route are pretty chummy with frisk. They are very tired of the resets regardless though (as it takes forever to land on the normal configuration) and sometimes halfheartedly bets with frisk who the next sans is going to be. The resets where everything is normal except for one monster are especially soul crushing for both the Sanses and frisk. They were so close!!!!!
For the design of another chars as sans they might go out of their way in the assets and wear sans classic jacket to showcase that they are a sans and are aware of what's going on. So their normal outfits but with sans jacket on top.
Interesting info of probability of a normal config happening in the randomizer:
~58 enemy types so 3364 (0.000297265% of normal config)
~322 monsters total so 103684 configurations (0.0000096446896% of normal config)
(let me know if I did the math incorrectly :) )