One ancient source says that he possessed the Cap of invisibility.

A more common trope is the cap of invisibility.

Hades had to remove his Cap of Invisibility before he could be seen, even by his fellow gods.

The Cap of Invisibility concealed his precise form, even from the other gods nearby, but everyone could feel his presence.

The Cap of Invisibility (1912)

But Shawahi insisted on learning about his acquisition of the cap of invisibility before going further into the activities of the palace.

Hippolytus: Slain by his grandnephew Hermes with his sword and wearing the cap of invisibility.

Athena, the goddess of wisdom, battle, and handicrafts, wore the Cap of Invisibility in one instance during the Trojan War.

Perseus, the Greek mythic hero who helped establish the Twelve Olympians, was equipped with a cap of invisibility to kill Medusa.

Cap of invisibility (aidos kyneê in Greek) is a mysterious helmet or cap that possesses the ability to turn the wearer invisible.

The cap of invisibility has appeared in Greek myth: Hades was ascribed possession of a cap or helmet that made the wearer invisible.

Every day he would go out, having donned his cap of invisibility, and regale himself upon his subject's endeavours to cope with the hardships he had designed for them.

The Cap of Invisibility enables the user to become invisible to other supernatural entities, functioning much like the cloud of mist that the gods surround themselves in to become undetectable.

"I would like a wand to do transformations, a cap of invisibility, swift slippers to walk the air, a purse of boundless wealth, a talisman to compel the love of all, a mirror-"

The helmet Pluto receives is presumably the magical Cap of Invisibility (aidos kyneê), but the Bibliotheca is the only ancient source that explicitly says it belonged to Pluto.

In some stories, Perseus received the Cap of Invisibility (along with the Winged Sandals) from Athena when he went to slay the Gorgon Medusa, which helped him escape her sisters.

The Cap of Invisibility was not used to avoid the Gorgons' petrifying gazes, but rather to escape from the immortal Sthenno and Euryale later on after he had decapitated Medusa.

Cloaks of invisibility are relatively rare in folklore; although they do occur in some fairy tales, such as The Twelve Dancing Princesses, a more common trope is the cap of invisibility.

According to Aeschylus, Hermes gives them to Perseus directly, In a better-attested version, Perseus must retrieve them from the Graeae, along with the cap of invisibility and the kibisis (sack).

It is revealed in The Sea of Monsters that Percy carried a picture of Annabeth in his binder and that Annabeth had been tailing Percy with her cap of invisibility on.

In the Percy Jackson & the Olympians series by Rick Riordan, Annabeth Chase (a Daughter of Athena) received a New York Yankees baseball cap from her mother that was a disguised cap of invisibility.