I now know how EVERYONE feels when me and Himosheep start Hetaliaing (can that be a verb?) at everyone.
And... hrm. If we're going with Dangerous Canon Entities, I raise you Ramuthra or Nouda from the Bartimeus-verse. They literally warp reality itself with their presence, they're big, nasty, and want to eat everyone, and the only way to get rid of them is to send them back to the world from whence they came.
Ah, and Annihilation Lightning is stated to be LETHAL with a direct hit. And it took up most of the magic that Ari grabbed from around Hogwarts, and the magic from draining Harry and Not!Nanoha, plus ... more