This knife was originally a wedding present to Helen of Troy from her 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.
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 well in a fight.
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 more as a mirror. Piper brings it on the quest, and uses it to kill multiple Gegeines while Jason is fighting Enceladus.
Piper looks into Katoptris numerous times to catch glimpses of the future. She sees Topeka 32 and Jason riding Tempest against Percy, 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 on 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.
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 very 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 aren't always helpful and may show something the owner doesn't wish to see. The owner also has very little control over what the knife will show them.
- 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 doesn't seem to use it as a weapon until the fight to free her dad. Until then, she used it as Helen had.
|Personal Weapons:||Riptide | Annabeth's Knife | Backbiter | Frank's Spear | Hazel's Spatha | Katoptris | Nico's Sword | Thalia's Spear | Aegis | Maimer | Kronos' Scythe | Ivlivs | Master Bolt | Poseidon's Trident | Sword of Hades | Reyna's Spear | Hades' Staff | Juno's Gladius|
|Magical Items:||Annabeth's Yankees Cap | Helm of Darkness | Keys of Hades | Flying Chariot | Golden Apple | Greek Fire | Hermes' Multivitamins | Leo's Magical Toolbelt | Nectar and Ambrosia | Pandora's Pithos | Winged Shoes | The Golden Fleece | Stygian Ice Whistle|
|Spoils of War:||The Minotaur's Horn | Medusa's Head | Kampê's Scimitars | Nemean Lion's Pelt | Gorgon Blood | Cornucopia|
|Items:||Camp Necklace | Chameleon Armor | Daedalus' Laptop | Golden Drachma | Mark of Athena | The Pax | Video Shield | Wristwatch Shield | Golden Mango | Sibylline Books|
|Blessed Metals:||Celestial Bronze | Imperial Gold | Stygian Iron|