What is Katoptris?

Katoptris is the name of the dagger that once belonged to Helen of Troy, the most beautiful woman in the world, who was favored by Aphrodite. Helen never used the knife as a weapon, but looked into its polished blade as a mirror instead. This is why she named it katoptris, which means 'looking glass.' Piper feels drawn to this weapon and chooses it to be her own. Later, she manages to use it as an actual weapon, spurred by the need to save the people she loves. She turns a beautiful trinket into something heroic through the power of love and friendship.