(Hey, if I make any mistakes please fix them, but please don't alter anything else. Also if you know any other ways to present the information better please do P.s- nothing was INTENTIONALLY copied)
InfectionTale is Au in which sans has gone missing do to an "incident" with Gaster. Papyrus and the others have been searching for him, but he is somewhere no one will find him, the void. Sans can leave the void through the door that appears in Waterfall, but he's afraid that he can't fit back in with everyone else. Though his appearance hasn't changed much, his mind is changed forever. His memories prior to The void are all gone.
Frisk in this universe wants to "save" Sans from Gaster, but since you must have a certain fun value to enter Gaster's room/the void, so she (in this universe frisk is a girl) kept resetting to get the correct fun value. Since the void is uneffected to the resets, Sans remained the same. Frisk is still trying to get the right fun value to save Sans.
Sans began losing his mind slowly over time. His mentality was that of a very deadly 12 year old child. His goal is to destroy everyone and everything that exists in the multiverse. He wants to make his own Multiverse.... his own reality... where he's not an outcast and has tons of friends, but for now... he's a phsyco murderer out for blood.
Sans lives in the void, instead of snowdin. Has Gaster in his head trying to "infect" him and take over his body.
Papyrus wears one of sans' hoodies and is a lot less optimistic, he also suffers from depression from time to time. Undyne has moved in with papyrus to keep him from growing lonely.
Toriel has exited the ruins to help search for sans, so she is also living with papyrus.
(sorry if it's kinda short, please forgive me. I'm still working out some kinks.)