Galactus, Eater of Worlds

When Galactus, Eater of Worlds enters the battlefield destroy all opponents creatures for the next 5 turns.
{t}{u/b}{u/b} Destroy all creatures and put the top 10 cards of target players library into the graveyard.
If Galactus, Eater of Worlds is on the battlefield for 5 turns, he goes supernova ( His power and toughness are triple your life total )