Sengir Doppelganger

Flying
Upon entering, put a -1/-1 counter on any creature on the battlefield. Doppelganger acquires all additional characteristics except color of that creature. During your upkeep, Doppelganger becomes a copy of a different creature in play instead. Put a -1/-1 counter on that creature. If no more creatures can be copied, bury Doppelganger.