Brineborn Cutthroat 
Creature — Merfolk Pirate
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.
Legality
standardLEGALpioneerLEGALmodernLEGALlegacyLEGALvintageLEGALcommanderLEGAL