Slifer, the Sky Dragon

Flying, Vigilance, Hexproof
Slifer enters the battlefield with X +1/+1 counters on
it, where X is the number of cards in your hand.
Whenever you draw a card, put a +1/+1 counter on Slifer.
Whenever a creature enters the battlefield under an opponent's control, it gets -3/-3 until Slifer leaves the battlefield.