Mana-Bound Mercenary

This creature enters the battlefield with X +1/+1 counters on it.
Pay off (During each opponent's turn, that
player may pay X, where X is 1 plus this creature's power. If they do, they gain control of
it, untap it, then remove all +1/+1 counters from
it and put X +1/+1 counters on it.)