Once upon a time there was an AU.
It was a strange AU that was, in a way, a mixture of Underfell and Horrortale, with it's own modifications.
In this AU, about 3 years after the human fell, the Sans of that AU and Frisk were having a pleasant discussion when Frisk challenged Sans to do the impossible: destroy a whole AU. You see, Sans had claimed he could do anything out of trying to impress Frisk. He readily agreed, wanting to do anything to reach his human's high standards. He started with Snowdin, then branched out to the Hotlands and then the Ruins until everyone was gone...
Except for Frisk.
You see, Sans had grown to despise Frisk for this challenge and went to find her. He eventually did so, and proceeded to obliterate her. He had succeeded. Everyone was gone..
Or so he thought.
Asriel, in the midst of the chaos, fled to Gaster's void. Gaster had promised protection if Asriel took up science in his place, and Asriel did so.
So, in a bout of panic, Asriel went to consult Gaster. Gaster, however, used Asriel to get his science work done and figure out how to fuse souls and powers.
Gaster somehow fused with Asriel, and used his newfound power to exit the void. With the mixed powers of Gaster and Asriel, Gaster managed to control Sans and was the only one who could overpower Sans.
And, in the end, Bloodbath and Gasriel were formed.