Obelisk the Tormentor

Indestructible, Menace, protection from white and from green
As long as your devotion to black is less than 6, Obelisk isn't a creature.
Once during each of your turns, you may sacrifice two creatures and pay 2 life to put that many +1/+1 counters on Obelisk the Tormentor. When you do, creatures your opponents control get two -1/-1 counters until end of turn. If an opponent's creature would die this way, exile it instead.