Whisperwood Elemental is a mythic Creature — Elemental card from Murders at Karlov Manor Commander (MKC), collector number 194. Current market price: $0.27, TCGplayer market prices via Scryfall.
At the beginning of your end step, 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.) Sacrifice this creature: Until end of turn, face-up nontoken creatures you control gain "When this creature dies, manifest the top card of your library."