Oh,
Oh, In that case..Do what daelin already suggested. Aww..man, can I type it out in Words, because I dunno how to work with trigger boxes or etc.
Events
Unit casts an ability(Exorcism is a kind of non-target spell right?)
Conditions
If ability cast equal is equal to (Exorcism)
Actions
Pick every unit within range of (Target Point of Ability Being Cast), Matching conditions or something like that,
If - Conditions
((Matching Unit) is Undead equal to true)
Actions - Deal (Amount) of Spell Damage to picked units.
If - Then
Actions - Do nothing.
I'm sorry I think I screwed up.:S I'm not exactly the triggerer. It's something like that. Warnico/Daelin or some good triggerer help please? Just don't follow my way. It's something like that but I'm sure I messed it up somewhere. ^^"