Well, i just think about some sort of boss spell, but since you seems didn't want to create such complex spell, i'll downgrade it to hero's spell: the hero create a big space holder (prefreable a unit, some sort of tornado like, that suck evryone to the center) and hold any unit that dare to come near 600 aoe from the space holder, the holded unit will take 4% of his max hp + 9% of his mana as damage (irgone the non mana unit)