Annabeth Chase's Yankees Baseball Cap is a magical item that has the ability to make the person who wears it invisible. As of The Mark of Athena, the magic cap no longer works because of Annabeth's fight with Minerva.
HistoryHades' Helm of Darkness, however it does not have any fear inducing properties. It has had many life-saving uses, quite notably helping Percy escape from the Furies on a bus in The Lightning Thief. Annabeth had used the cap to defeat the Laistrygonians, when they attacked Percy and Tyson in the Sea of monsters. She used it to trick Polyphemus in The Sea of Monsters, escape from Mount Saint Helens in The Battle of the Labyrinth and fight alongside Percy in The Last Olympian. So far, Percy is the only known demigod other than Annabeth who has used this cap.
In battle, or in any circumstance as a demigod, the hat is very useful. When worn, it makes the user and any item they're holding invisible. That in and of itself would make it very powerful, as its usefulness of stealth is an important and useful factor in battle and ingenuity on Annabeth's part makes it invaluable. It loses its magic in The Mark of Athena, after she fights with her mother.
- Annabeth Chase – The cap belongs to Annabeth, and given to her by her mother Athena. The cap helped cause many victories and success in Annabeth's and Percy's quests.
- Percy Jackson – When Annabeth was abducted in The Titan's Curse, Percy kept the cap and used it to eavesdrop on Atlas and Luke. Six months later, he used it to sneak up on Kronos before he could reform. He also used it briefly in The Lightning Thief and The Battle of the Labyrinth.
- In ancient myths, Athena gave Perseus a helmet that turned him invisible. During the series, Annabeth, a daughter of Athena, has offered to let Percy (Perseus) use it on multiple occasions.
- In her picture Annabeth is wearing her hat, but in the books, when she has it on she's invisible.