Polukranos, Thero's Hydra

When Polukranos enters the battlefield, you
may pay {3}. If you do, exile 2 target creatures you don't control
{1}{g}{b}{g}{b}: Destroy target legendary creature until Polukranos is exiled.
{3}{g}{b}: Target player loses X life, where X
is their amount of creatures on the battlefield.