New AU!
Wooot
I love the name so much, when someone suggested this to me I was happy.
I love the thought of reincarnation into beasts.

Anyway....
Here's the backstory:

When monsters lost the war to the humans, they were cast underground.
Following behind them, a disease.
When a monster was born, the disease were to kill them immediately, as a side effect to the disease,
Every family was struck with grief due to this, so a cure was made.
But it wasn't exactly the cure they wanted.

Every monster was injected with the cure to fight off the disease, and hopefully save the monster kind from dying out.
But, a beast started to grow inside them. It fought off the disease, but grew stronger, eventually corrupting the body, taking control.

No known cure is for this, except for every monster to absorb a certain amount of human souls EACH, to become pure and healthy once again. The only way to save every monster is to be freed, but very few humans ever fall down, and every monster's existence is coming to an end.