Master of the Pearl Trident is a special Creature — Merfolk card from Time Spiral Remastered (TSR), collector number 310. Current market price: $2.35 (foil $53.60), 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.)