False Hydra

Hexproof, cannot attack or block.
This Creature's power and toughness is equal to the amount of all creatures in graveyards and enters the battlefield under an opponent's control.
At the Beginning of Opponent's upkeep, they sacrifice a creature they control. This Creature gains +1/+1 equal to that creature's converted mana cost. If no creatures were sacrificed, Opponent takes damage equal to this creature's power.