Mastery of the Unseen is a rare Enchantment card from Ugin's Fate (UGIN), collector number 19. Current market price: $1.00, TCGplayer market prices via Scryfall.
Whenever a permanent you control is turned face up, you gain 1 life for each creature you control. {3}{W}: 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.)