Restless Reef is a rare Land card from The Lost Caverns of Ixalan (LCI), collector number 349. Current market price: $1.92 (foil $2.73), TCGplayer market prices via Scryfall.
This land enters tapped. {T}: Add {U} or {B}. {2}{U}{B}: Until end of turn, this land becomes a 4/4 blue and black Shark creature with deathtouch. It's still a land. Whenever this land attacks, target player mills four cards.