Katoptris (meaning "looking glass" or "mirror" in Ancient Greek) is a dagger that once belonged to Helen of Troy, fitting accurately for its name as Helen only used it as a mirror. However, in The Lost Hero, it became Piper McLean's dagger after she selects it to be her own, and she uses it for battle as well as seeing visions of the future in it.
This knife was originally a wedding present to Helen of Troy from her first husband Menelaus, and when looked upon, can show much more than the holder's reflection (as shown when Piper sees her father tied to a pole by the giant, Enceladus). Helen used the dagger as a mirror, which is why she named it Katoptris, meaning "mirror" or "looking glass." It was not used as a weapon when Helen was the owner, but as a looking glass so that she could gaze upon her beautiful reflection.
Katoptris is a parazonium blade. It was mostly ceremonial, carried by high-ranking officers in the Greek armies. It showed that the wielder was a person of power and wealth, but it could be used in a fight as well.
Piper finds and chooses it when Annabeth Chase takes her to the Armory. Annabeth tells her that the knife has probably never seen battle, as Helen mostly used it as a mirror. When Piper first looks at it, it shows the giant Enceladus and her dad tied to a stake beside a purple fire. Piper brings it on the quest, and uses it to kill multiple Gegeines while Jason is fighting Enceladus and at the Wolf House.
Piper looks into Katoptris numerous times to catch glimpses of the future. She sees Topeka 32 and Jason riding Tempest against Percy Jackson, her mother Aphrodite, the river god Achelous, Bacchus offering her a silver goblet, a symbol for his help, and Jason, Percy and herself drowning in black oily water. The last vision Piper sees is the romans planning an invasion of Camp Half-Blood in Manhattan, setting up base in Central Park. After being startled by the visions, Piper decides to leave Katoptris with Coach Hedge, who looks up baseball scores through it and keeps an eye on them from the ship. Later, Leo, Hazel, and Frank are rescued by Coach Hedge after they hack into Coach Hedge’s favorite satellite channels and tell him to bring the ship around to rescue them.This blade then shows Piper some distracting and disturbing images.
Piper brought Katoptris with her while she, Apollo, Meg, and Grover were heading towards the entrance to the Burning Maze in the Hall of Records near Grand Park in downtown L.A.. Grover inquired about the dagger, to which Piper confirmed that it was indeed Katopris and that she found it in an abandoned toolshed at Camp Half-Blood. This led Apollo to think about how time lays waste to everything eventually, and that someone might find him in a toolshed, thousands of years later. Piper also mentioned Katoptris stopped showing her visions last summer and asked if it was due to Apollo getting kicked out of Olympus.
Piper later throws Katoptris at Medea's sun dragons. It impales the left dragon's eye, causing Medea's chariot veering off course. Medea, when face-to-face with Piper, compliments the weapon's appearance but says that it's only pretty and not powerful.
Piper uses her dagger for the last time in the series to stab Medea in the Sibyl's room in the Burning Maze, saying "one good stab in the back deserves another."
It is sheathed in worn black leather and is bound in bronze. Katoptris has a hilt made of polished wood with an eighteen inch triangular blade made from Celestial Bronze.
- Being made of Celestial Bronze, Katoptris is effective against monsters.
- The blade of the knife can be used as a mirror.
- Katoptris has the ability to show the owner visions. However, these visions are not always helpful and may show something the owner does not wish to see. The owner has very little control over what the knife will show them.
- It is mentioned that it is a ceremonial dagger, so it may be used just for decoration or specific ceremonious roles.
- Katoptris is Greek for 'Mirror' or 'Looking Glass.' Annabeth speculates that Helen, its original owner used it only for this purpose and nothing else.
- Although it is considered a weapon, it was never really used in battle by Helen.
- Piper does not seem to use it as a weapon until the fight to free her father. Until then, she used it in the same way as Helen had.
- After the events of The Blood of Olympus, it is shown that the blade has lost its powers of foresight. Whether this is because Apollo lost his influence over the voice of Delphi is unknown. The subsequent destruction of Python and the restoration of the Oracles may have restored the dagger's powers, but this is unconfirmed.