I Think This is Cool. Is it Cool?
There's a girl who's not human, she's fairy or nymph or some kind of ancient spirit, but she looks exactly human. She doesn't even have an obvious distinction, like a strange aura or different-colored eyes. She is small and not athletic, but if you tell her who you are she has the power of life and death over you. It's not knowing who you are, but the fact that you told her who you are. She can kill you immediately or years later, but from that moment any time she's in the same room as you, you might just fall down dead. If you look at her during the moment that she chooses to kill you, you will see her change into a monster, but nobody else will, and it's not the monster who killed you. The coroners will be baffled, because there are no marks, you just stopped existing, and the only way you could have prevented it was to lie when you introduced yourself.