Primordial Mist is a rare Enchantment card from Duskmourn: House of Horror Commander (DSC), collector number 123. Current market price: $0.33, TCGplayer market prices via Scryfall.
At the beginning of your end step, you may 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.) Exile a face-down permanent you control face up: You may play that card this turn. (You still pay its costs. Timing rules still apply.)