A Sans AU Is a term used in the Undertale community to represent fangames that take place in an Alternate Universe which heavily focus on 1 or more Sans fights.
They often are extremely edgy for no reason and treat fanon as canon information.
Timmy: "Have you seen that Sans AU where Sans goes insane and kills everyone?
Alex: Which one?"
Timmy: The one that's nearly impossible and has been played by tons of Youtubers"
Alex: "That doesn't narrow it down"
by PROPLAYEN August 8, 2022