This is an Undertale AU following a Genocide Route, where Gaster has gone insane, and has murdered the entire Underground, except for a few survivors.
After Frisk decided to forget their life on the surface, in favour of a life with their new, best friends. W.D Gaster was brought back into the Underground, but his time in the void had driven him insane, because when as soon as he stepped out, and stared at the warming faces around him, he quoted that all Monsters and Humans must be obliterated.
With that, he begun a mass Genocide on the Underground, being merciless towards every living organism. Among the death toll, which was in the millions, were Mettaton, Napstablook, Asgore, Toriel, Alphys, and even Undyne. Gaster had, at this point gained so much LOVE, that he was basically a Demon.
Soon though, Gaster went too far, and murdered Papyrus and Frisk right in front of Sans' eyes in the judgement hall, whem the three tried to stop him. After Gaster left Sans alone, commenting that he "wasn't worth his power," Sans took Papyrus' now dust-covered scarf, and picked up Frisk's body, before burying it at the barrier, along with Papyrus' battle body. He took Frisk's soul, along with the other 6 Human Souls, and absorbed them.
Sans swore with an oath, to try and protect the barrier at all costs, so Gaster doesn't get through to the Human World. And with the Annoying Dog and Flowey at his side, Sans will wait for the day.
The day he will get revenge.
The day where he will save what's left of the Underground.
The day of Judgement.
Sans- The main protagonist of Survivortale. He loves puns and drinking barbecue sauce, and he used to take frequent naps, but he rarely sleeps at all nowadays. Sans is aware of the resets, and surprises Flowey with his knowledge sometimes. He is also aware that he lives in an Alternate Universe, and that there are countless more out there.
He looks quite a lot like the original Sans, but instead wears a black T-shirt with a yellow circle in the middle. He also wears a light blue winter jacket, with a white stripe on the right side of the chest. Sans also wears blue and white sneakers, and has a cracked tooth.
Powers- These are the standard Sans attacks, but thanks to the 7 Human Souls, his powers and dodging have increased significantly.