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- " Don't Hurt Anyone..." by HeavenFell
Heavenfell is an Undertale AU 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.
[Plot summary of Heavenfell needed]
- Monsters are violent and lethal instead of kind and compassionate
- Flowey befriends and assists Frisk in traveling the Underground
- Papyrus is competent and already a Royal Guard, and is abusive towards his brother Sans, whom he treats as an underling
- Asgore holds the "Kill or be Killed" mentality instead of Flowey, and rules the Underground with an iron fist