Perseus, King's True Form

{10}{b}{g}: Monstrosity 10. This ability costs 1 less to activate for each creature with power 4 or greater your opponents control. (If this creature isn't monstrous, put ten +1/+1 counters on it and it becomes monstrous.)

When Perseus, King's True Form becomes monstrous, destroy all artifacts and creatures other than Perseus, King's True Form.