Undertale: Desolation

Reset after reset, the underground's scent had been tainted with the stench of a mild stale substance known as dust. Frisk had finally decided to stop doing the genocide. With what seemed like one final reset, they had completed a true pacifist route.

Unbeknownst to them, a certain ghostly child has decided that Frisk doesn't deserve to have their happy ending. Chara, with the last amount of energy they had, created something... or someone that Frisk would NEVER forget for the rest of their days.