Brineborn Cutthroat is a uncommon Creature — Merfolk Pirate card from Game Night: Free-for-All (GN3), collector number 24. Current market price: $0.29, 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.