A Killer Among Us is a uncommon Enchantment card from Murders at Karlov Manor (MKM), collector number 167. Current market price: $0.16 (foil $0.18), TCGplayer market prices via Scryfall.
When this enchantment enters, create a 1/1 white Human creature token, a 1/1 blue Merfolk creature token, and a 1/1 red Goblin creature token. Then secretly choose Human, Merfolk, or Goblin. Sacrifice this enchantment, Reveal the creature type you chose: If target attacking creature token is the chosen type, put three +1/+1 counters on it and it gains deathtouch until end of turn.