Mastery of the Unseen is a rare Enchantment card from Murders at Karlov Manor Commander (MKC), collector number 74. Current market price: $0.15, 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.)