Cap of invisibility

In Greek mythology, the Cap of Invisibility (Ἄϊδος κυνέην (H)aidos kuneēn in Greek, lit. dog-skin of Hades) is a helmet or cap that possesses the ability to turn the wearer invisible. Also known as the Cap of Hades, Helm of Hades, or Helm of Darkness. The helm was used by numerous figures, including the goddess of wisdom, Athena, the messenger god, Hermes, and the hero, Perseus. The Cap of Invisibility enables the user to hide from the eyes of other supernatural beings, functioning much like the cloud or mist that the gods surround themselves in to become undetectable.

Origins
According to the mythographers, the Helm of Darkness, amongst other items, was created by the Uranian Cyclops during the War of the Titans (Titanomachy). The Cyclops gave the thunderbolt to Zeus, the trident to Poseidon, and the Helm of Darkness to Hades. Other than this, however, Hades never used the Helm of Darkness, nor was it said that any of the users of the Helm ever borrowed it directly from him.

Athena
The goddess of wisdom, battle, and handicrafts, Athena, used the Cap of Invisibility in one instance during the Trojan War. She used it to become invisible to Ares when she aided Diomedes, his enemy. Her assistance even enabled Diomedes to injure the god of war with a spear.

Hades
The Lord of the Underworld, Hades, wore the Cap of Invisibility in the Titanomachy, or divine war in which the gods Zeus, Poseidon, and Hades were given special tools by Cyclopes to aid them in their battle against the elder gods. The night before the first battle, Hades put on his helmet and, being invisible, slipped over to the Titans' camp and destroyed their weapons, ultimately giving the younger gods victory in this battle. The war lasted for ten years and ended with the victory of the younger gods.

Hermes
The messenger god, Hermes, wore the Cap during his battle with Hippolytos, the giant.

Perseus
In some stories, Perseus received the Cap of Invisibility (along with the Winged Sandals and a silver wallet) from Hermes when he went to slay the Gorgon Medusa. In other myths, however, Perseus obtained these items from the Stygian nymphs. 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.