I thought of an AU! Please tell me if it's been thought of or has something very similar. ovo
An unknown disease strikes the Underground. (Needs first victim). Alphys studies the disease, and the process starts as the monsters falling in a deep comatose-like state where the disease has time to feed and grow. With the weakened state of body, the disease can grow more rapidly. What grows inside is more like an infection or tumor, which leaks black liquid, causing black liquid to drip from (and is not limited to) eyes, nose, mouth, ears, and any other cracks or holes. The tumor is more like a parasite and 'eats' the magic within the monsters, slowly killing them, but at the same time acting like a life force so they are in a near-dead state, or undead. Skin/bones become discolored, fur falls off, parts get weaker and more susceptible to breakage. Since monsters don't decay and are made of magic, they take on a melted amalgamate-like appearance. Once the disease fully takes over, it has a mind of its own and the monsters are controlled by the parasite. However, they can still think, but are too weak to move themselves. They can, however, still speak a few words - for example, 'help me'...
Kind of a horror-like thing
I imagine diseased sans walking up to frisk saying 'help me'... and I'm liking it so far .D.
oh yeah, and my other AU ideas
Flowertale-
All monsters and maybe Frisk and Chara are replaced with flowers, while Flowey is a human/monster.
Warriortale-
All monsters and humans wear cool battle armor. Needs better description.
Mirrortale-
Alternate versions of selves(?). Needs more info.
Surfacetale-
Needs better name. Like Undertale but Frisk's role is replaced by Sans and he goes up to the surface.
Kingtale-
Frisk becomes king after Asgore dies/Asriel lives and becomes king after Asgore dies.
