UnderBound (Undertale AU) [On Hold]

A pacifist ending means everyone made it out of the underground alive. A happy ending. Closure.

Except for two particular monsters. Two particular skeletons.

In a timeline where Frisk waited just a little while before freeing everyone from the underground, they are approached by a peculiar Lost Soul begging them to bring Sans back to them. They guide Frisk to a certain mysterious room in Sans' home and chaos soon ensues.

Throw in an alternate dimension, a deleted scientist, and three friends just trying to get home, and it seems like the return to the surface may have to wait just a bit longer.

At least, until a dirty hacker breaks through and teams up with a frightening Lost Soul of his own. Then Frisk realizes that the adventure has become a race for their and Sans' lives.