Ugin's Mastery (53) [Murders at Karlov Manor Commander]

Whenever you cast a colorless creature spell, manifest the top card of your library. (Put it onto the battlefield face down as a 2/2 creature. Turn it face up any time for its mana cost if it's a creature card.) Whenever you attack with creatures with total power 6 or greater, you may turn a face-down creature you control face up.