A Killer Among Us 
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Enchantment
When A Killer Among Us enters the battlefield, 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 A Killer Among Us, Reveal the chosen creature type: If target attacking creature token is the chosen type, put three +1/+1 counters on it and it gains deathtouch until end of turn.