Dustbelief is an AU of the Dusttale series. In this alternative storyline, Dusttale!Sans attempts to kill his brother. He fails and instead is killed by Papyrus. The EXP earned in that single moment overloads Papyrus, driving him insane. He decides that the human is responsible for Sans's actions and subsequent death so resolves to get revenge, somewhat similar to the Disbelief Papyrus AU. This combination is where the AU gets it's name.