Master of the Pearl Trident is a rare Creature — Merfolk card from Duel Decks: Merfolk vs. Goblins (DDT), collector number 9. Current market price: $0.65, TCGplayer market prices via Scryfall.
Other Merfolk creatures you control get +1/+1 and have islandwalk. (They can't be blocked as long as defending player controls an Island.)