Aatrox, The Darkin Blade

Aatrox enters the battlefield as a legendary artifact creature with an equip cost of {4}. The equipped creature gains +3/+3.
Subjugate - Sacrifice the equipped creature, Aatrox becomes an 8/8 legendary creature, with a number of counters on him equal to the sacrificed creature's toughness. Remove one of these counters at the end of each of your turns, when there are no counters left Aatrox returns to being a legendary artifact creature.