'deal damage when the opponent is about to heal'?
It sounds good, but it comes with a few problems.
The main one is that the opponent knows you have this card (permanent?) out. Even if you use cloak, the opponent sees what you play.
Secondly, it requires either a very low damage (making it worthless vs healing spells), or to be destroyed on use. A third option is to let it last a limited amount of turns.
If it heals too much, you may want to antimatter your RoL army, causing the ai not to target them.
finally, what happens if the opponent uses SoSac? will it improve it's effectiveness?
It sounds like a good idea, though.