Kethek, Crucible Goliath is a rare Legendary Creature — Phyrexian Beast card from Phyrexia: All Will Be One Promos (PONE), collector number 206p. Current market price: $0.40, TCGplayer market prices via Scryfall.
At the beginning of your end step, you may sacrifice another creature. If you do, reveal cards from the top of your library until you reveal a nonlegendary creature card with lesser mana value, put it onto the battlefield, then put the rest on the bottom of your library in a random order.