Mortem, the Willowed Watcher

Mortem, the Willowed Watcher
cannot attack or block.
During each player's upkeep, you get to look
at target opponent's hand and choose a card.
Once the chosen card is cast, target player
must sacrifice a creature he or she controls.
Once 10 or more creatures are sacrificed this way,
Mortem, the Willowed Watcher can attack and
block.