To be used, the Key must be pure, e.g. not a vampire/monster/whatever. What if the monks of the Anitaverse decided to make the Key useless to The Beast/Glory while they hid it?

Instead of making Dawn a fourteen-year-old damsel in distress, they make her a fourteen-year-old monster. Whether she's a vampire or a wereanimal, or something else entirely, is up to you. If you want to make her something a little more "human" to muddy the waters, she could be a necromancer, someone with psychic abilities (say a touch clairvoyant or telepath), a magic-user of some kind, suffer from Vlad's Syn...

"I am not your human servant. I will never be your human servant. I wish you'd just accept that and leave me the fuck alone."
-- Anita Blake to Jean-Claude, Circus of the Damned, Chapter Six.

I'd love to see a story where a Vengeance Demon grants Anita's wish. How does it change the history of the Anitaverse?

General Gregor: What do you know of The Beast?
Buffy: Strong, fast, hell-god.
General Gregor: From a dimension of unspeakable torment.
Buffy: A demon dimension. I know. She ruled with two other hell-gods, right?
General Gregor: Along with The Beast, they were a triumvirate of suffering and despair, ruling with equal vengeance. But The Beast's power grew beyond even what they could conceive, as did her lust for pain and misery. They looked upon her, what she had become, and trembled.

Triumvirate. Absolute power corrupting absolutely. Glory/The Beast = Anita. Have fun.