Can I make an Exorcism spell?

I want to take the Holy Light ability and make it so it only affects Undead and so it only works on enemies. I took the other half to make a Healing spell too, but for some reason the game thinks the abilities are the same when they are on the same unit. It also seems to ignore my target settings.

Is there any way to make a Holy Light-esque spell that only harms Undead, and a Holy Light spell that only heals?

Well for one, you can't have two of the same abilities on one unit, custom or not. If they have the same OrderID, they'll bug.

The problem is that both abilitites share the same orderid, so clicking either would just cast teh first one listed in the object editor.

You need to base your second spell off of a different ability. Channel might work.

Try using a modified chain lightning (for damage)/healing wave (For healing) spell and then check the targets. If the targets are incorrect, then display an error message and tell the unit to stop.

erm, just use triggers?
like..unit casts a spell, target is undead, deal X damage, else do nothing..blablabla
and to heal modify the heal spell? or any spell for that matter and use triggers to heal?
oh and for the undead spell, make it not cast at all if the target is not undead..and add an error message, if this is confusing you ill try to explain it better