Mary O'Kill is a rare Legendary Creature — Human Villain card from Unstable (UST), collector number 138. Current market price: $0.17 (foil $0.55), TCGplayer market prices via Scryfall.
{1}{B/R}: Switch a Killbot or Mary O'Kill in your hand with one on the battlefield. (If a creature is tapped, the switched creature is tapped. The same is true for untapped, attacking, blocking, enchanted, equipped, and targeted. Any counters on a creature are on the switched creature instead.)