[Taqwus]

Perhaps the most damaging, inane spellcasting I've had was a communion master of some sort (a while ago, don't even recall the nation I was playing, but probably Pythium) casting Phoenix Pyre -- which carries over to the communicants as well -- and then promptly falling over dead, possibly from previously taking a poison hit or something like that.
Boom. Explosion hurts those nearby, including communicants. Boom. Boom. More boom. Boom. Aiiiiiiie!