Risen Reef is a rare Creature — Elemental card from Secret Lair Drop (SLD), collector number 1257. Current market price: $14.35 (foil $15.82), TCGplayer market prices via Scryfall.
Whenever this creature or another Elemental you control enters, look at the top card of your library. If it's a land card, you may put it onto the battlefield tapped. If you don't put the card onto the battlefield, put it into your hand.