Petrifying Drifter

When Petrifying Drifter enters the battlefield,
each opponent puts the top three cards of their
library into their graveyard.
Petrifying Drifter's power is equal to the total
number of creature cards in your opponents'
graveyards.
As long as there are five or more creature cards
in an opponent's graveyard, Petrifying Drifter
has flying.