Massacre Girl is a rare Legendary Creature — Human Assassin card from War of the Spark (WAR), collector number 99. Current market price: $0.52 (foil $1.52), TCGplayer market prices via Scryfall.
Menace When Massacre Girl enters, each other creature gets -1/-1 until end of turn. Whenever a creature dies this turn, each creature other than Massacre Girl gets -1/-1 until end of turn.