Myriyan, the earth shaker

Flying, First strike
When Myriyan, the earth shaker enters the battlefield,
you may pay {x}{x} mana. If you do, create that
many 2/2 blue tornadoes.
Pay {x}{x} mana. Create that many 2/2 blue tornadoes.
Myriyan, the earth shaker's power is equal to the
number of 2/2 blue tornadoes there are on the battlefield.