Supertale is an AU where Frisk never achieved a Pacifist Ending. Frisk kept resetting and resetting. This made Sans very mad at Frisk. So, one day, Sans killed Frisk and took their SOULl, giving him so much DETERMINATION that he went insane.
Supertale's whole story is that one day, Frisk fell into the Underground, met Flowey, left the ruins and stuff. But, when Sans went up to Frisk to do the whoopie cushion trick, he felt a wave of power pass through him.