Primetale

Primetale is a au by YoghyHam
The main prologue is:
+Gaster needed the original timeline destroyed so that  there could be "balance" (not really)
 but when he destroyed everything but himself, the timeline started to crumble from lack of suspension, to counteract this, +Gaster attempted to create a new Sans with enough power to sustain the timeline. (since he was +Gaster was not from the "Prime" timeline, he could not sustain it himself.) when he created this new sans (Xinx)  -Gaster sensed that the Prime timeline had been disturbed, and quickly made two young skeletons each made in the IMAGE of sans (but not made from the essence of sans, unlike Xinx) Vinerion for strength and cunning, and Ashia for intelligence and smarts but UNLIKE +Gaster, -Gaster did not give the two skeletons specific personalities, she decided that she will let their personalities develop on their own time, (but she DID make sure that there was good in their hearts) she and her two creations then set off to destroy +Gaster, when they arrived she saw that +Gasters creation had such powerful magic that If +Gaster were to tell Xinx to kill -Gaster, -Gaster would die in a heartbeat, so then she left her creations to talk with XInx, and try and free him from +Gasters control, meanwhile she and +Gaster, went somewhere else to duel it out, but sadly, +Gaster easily Killed -Gaster. meanwhile Ashia and Vin, were able to successfully  Talk to, and free Xinx. but what +Gaster made XInx do, well, it left him with  those memories for the rest of his almost infinite life. so they went to confront +Gaster. 
Ashia and Vin were fueled by Justice, And Xinx was fueld by REVENGE, they successfully destroyed +Gaster. but without him, the timeline slowly started to crumble, so the three of them escaped to a nearby Post - Pacifist  timeline where they are today.