Wow, very interesting! It certainly makes a lot of sense.
The ATK/HP should be -, not 0/0. Although a 0/0 creature would be interesting...
Anyways. As dragonhuman said, it's quite situational. I think I'd expand it a little to encompass all Undead.
"Every Vampire, Lycanthrope, Mummy, Skeleton, and Bone Dragon in play is turned into Ash."
Might be a bit complicated. Because there's a card to buff Death and Darkness, why not a card to nerf it?
"Every Death and Darkness creature on the field is turned into Ash."
Needs to cost more, though.