Brineborn Cutthroat is a uncommon Creature — Merfolk Pirate card from Core Set 2020 (M20), collector number 50. Current market price: $0.17 (foil $0.39), TCGplayer market prices via Scryfall.
Flash (You may cast this spell any time you could cast an instant.) Whenever you cast a spell during an opponent's turn, put a +1/+1 counter on this creature.