Slifer, The Sky Dragon

This spell cannot be countered.
When Slifer enters the battlefield, draw a
card, then sacrifice three creatures or sacrifice Slifer.
Indestructible, hexproof, flying, menace and haste.
When Slifer attacks, draw a card.
Slifer's power and toughness are both equal to your
cards in hand +3.
Whenever a creature enters the battlefield under an opponent's control, it gets X -0/-1 counters,
where X is half of Slifer's power (rounded up).