Heavenfell is an AU created by Castel. Similar to the original Undertale from Toby Fox, Frisk has fallen to the underground and has to make their way to the portal. Unlike Underfell, Frisk and Flowey team up doing the Pacifist run in the Genocide Run world with the monsters will try to kill them.
Their way through the underground, they see their own health degrading and black marks running over their body without finding the reason why nor the source of these. They just know or feel they don’t have much time until they are corrupted too, so they need to be fast, no matter how many times they’ll have to die and the suffering they’ll have to go through.