Gyai, Terror of the Depths

As long as the number of cards in your graveyard
is equal to or greater than Gyai's toughness, if an
opponent would mill one or more cards, they
mill twice that many cards instead.
{2}{u}{b}{b}, {t}: Exile another target creature and
Gyai. That creature's controller mills cards equal
to that creature's toughness. Return Gyai to the
battlefield under its owner's control at the
beginning of the next end step.