I am not sure how I feel about this...
The wording of "ATTACK" dmg is confusing. Do they actually mean SKULL dmg? Or ANY dmg done while "attacking"?
If it is SKULL dmg then I don't think this is a problem. Its annoying, its "going down the rabbit hole" as was mentioned above, but its a single shot and then its over, though paired with a well placed KoS, or lucky BD cast and it could be devastating.
If however it is ANY dmg then it can be entirely OP, even if it's just a one time cast... (I personally, do not believe that it will be this.)