Xenagos, God of the Wilds

Indestructible
Whenever another creature enters the battlefield
under your control, you may have that creature
gain haste and get +X/+X until end of turn,
where X is that creature's power. If you do, sacrifice that creature that the beginning of your next turn.
Xenagos' power and toughness are each equal to
the greatest power among creatures you control.