EXORCISM spell

EXORCISM spell

I am having quote some trouble figuring out how to do an exorcism spell, such as when the hero casts it all undead in 500 range of the hero take a certain amount of damage. I am not sure how to do this with triggers and if someone can help me with it that would be awesome. Also to note there is not "Undead" category for units allowed to be targeted.
Also as another note, the spell is a unit spell, not a hero spell. So only 1 level.

EVENT-
unit starts the effect of an ability
CONDITION-
ability being cast = *YourSpell*
ACTIONS-
Pick every unit within 500 of triggering unit matching Matching Unit is owned by an enemy of triggereing unit and Unit class is undead and do Set life of matching unit = life of matching unit - *HOW MUCH DAMAGE YOU WANT* 

and yes, the is just one action.

@Iplaystarcraft: Doesn't life reduction give bounty problems?

@Discordus: This spell was made before, 100% sure by Vexorian in Caster System, maybe someone else done it before.