Shriek in terror and revert to their fetal graboid state? ... j.k.
The more I think on it, the more I realise that this is actually quite a tricky card to balance properly.
Mainly because the damage output could vary so greatly, even compared with other mass CC.
Does anyone know if there is a page with details of average card usage?
looking at the attack power of each creature vs its average deck usage statistics may be of some help.
Basically, I'm thinking that the average damage output, based on the average attack power of creatures commonly used, can give a good gauge. From there the cost can get balanced against other Mass CC spells more easily.