Heavenfell is an UndertaleAU and a Sub-AU to Underfell created by Castel. Similar to the original Undertale by Toby Fox, Heavenfell begins with Frisk having fallen into the Underground and needing to cross the Barrier to return to the surface. Following that, this AU's plot closely follows that of Underfell, with Frisk and Flowey teaming up as pacifists trying to befriend the monsters of a twisted world where it's kill or be killed.
Unlike Underfell, as Frisk travels through the Underground they develop an illness that causes black marks to form all over their body. Though they don't know what it is, Frisk feels they only have a limited time to escape the Underground before succumbing to this condition.