Ugin's Mastery is a rare Enchantment card from Commander Masters (CMM), collector number 751. Current market price: $1.21 (foil $2.20), TCGplayer market prices via Scryfall.
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.