Highjump Assassin

Emissary (At the beginning of your end step, another target player gains control of this creature.)
Whenever another player gains control of Highjump Assassin, put a +1/+1 counter on Highjump Assassin.
Whenever an opponent casts a spell while not controlling Highjump Assassin, counter that spell if its converted mana cost is less than or equal to the number of +1/+1 counters on Highjump Assassin.