Risen Reef is a mythic Creature — Elemental card from Special Guests (SPG), collector number 143. Current market price: $3.59 (foil $5.20), 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.