Shapeshifter's Marrow is a rare Enchantment card from Future Sight (FUT), collector number 58. Current market price: $0.51 (foil $2.92), TCGplayer market prices via Scryfall.
At the beginning of each opponent's upkeep, that player reveals the top card of their library. If it's a creature card, the player puts the card into their graveyard and this enchantment becomes a copy of that card. (If it does, it loses this ability.)