The Helm of Hades was born in the prison of Tartarus, the dark pit beneath the earth where the Cyclopes Brontes, whose name means Thunder, Steropes, whose name means Lightning, and Arges, whose name means Bright Flash, had been chained by Uranus, the sky, and then by Cronus, the Titan king. These one-eyed sons of Gaia, the earth, and Uranus were master smiths, and when Zeus, the youngest son of Cronus, freed them to fight in the Titanomachy, the ten-year war between the old Titans and the young Olympian gods, they forged three weapons of unimaginable power. For Zeus, who would rule the sky, they made the thunderbolt, a shaft of celestial fire that none could withstand. For Poseidon, who would rule the sea, they made the trident, the three-pronged spear that could shatter stone and summon waves. And for Hades, who would rule the dead, they made the helm, a cap of such cunning craft that whoever wore it became invisible, not merely hidden in shadow but utterly unseen, erased from the sight of gods and men alike.

Hades received the helm before the three brothers drew lots for the division of the cosmos. When the Titans fell and the war ended, Zeus took the sky, Poseidon took the sea, and Hades took the misty darkness of the Underworld, the realm where the dead wander in eternal shadow. The helm was the perfect symbol of his kingdom, for the dead are unseen by the living, and Hades himself was the most withdrawn of the gods, rarely leaving his throne in the dark halls beneath the earth. Unlike the thunderbolt, which Zeus hurled in battle and display, or the trident, which Poseidon used to reshape coastlines, the helm was a weapon of secrecy and stillness. Its power was not in destruction but in absence, in the removal of the self from the world of perception. When Hades did leave his realm, as he did when he rose to the surface to seize Persephone, the daughter of Demeter, goddess of the harvest, to make her his queen, he may have worn the helm himself, though the old stories are silent on the point. The very fact that he could move unseen made him the most feared of the three brothers, for a king who cannot be seen cannot be resisted.

The helm's most famous loan came generations later, when the hero Perseus was sent by the king Polydectes to fetch the head of the Gorgon Medusa, one of three monstrous sisters whose faces were so terrible that one look turned living things to stone. Perseus could not approach Medusa directly, for even her reflection in a shield might petrify him, and her sisters, Stheno and Euryale, were immortal and deadly. The gods came to his aid. Athena, the goddess of wisdom and war, gave him a polished bronze shield to use as a mirror. Hermes, the messenger god and patron of thieves, gave him a sickle of adamant to sever the Gorgon's neck. And from Hades himself, or from the nymphs who guarded his realm, Perseus received the cap of invisibility. He put it on and vanished from sight, walking unseen past the sleeping serpents that coiled at the Gorgons' feet, approaching Medusa as she slept, and striking off her head before she or her sisters knew he was there. Without the helm, the quest would have been impossible; with it, Perseus became the only mortal to slay a Gorgon and survive.

The helm appears again in the Trojan War, the ten-year siege of the city of Troy by the Greek armies, as recorded in Homer's Iliad, the great epic of the war. Athena, the goddess who favoured the Greeks, borrowed the cap from her brother Hades and walked unseen among the Greek tents. She found Diomedes, the prince of Argos and one of the greatest Greek warriors, and filled him with such battle-fury that he attacked the gods themselves who fought for Troy. Wearing the helm, Athena stood beside him unseen, guiding his spear, and together they wounded Ares, the Olympian god of war, driving him from the battlefield in pain and humiliation. The cap made it possible for a mortal to strike a god and live, for Ares never saw the hand that guided the spear. In this moment the helm revealed its full power: not merely to hide, but to make possible the impossible, to let the unseen tip the balance of heaven itself.

In later tales, Hermes, the messenger of the gods and guide of souls to the Underworld, was also said to possess the helm or one like it, though the sources disagree on whether it was the same cap or a copy. Hermes used it in his role as psychopomp, the conductor of dead souls, moving unseen among the living to lead the shades of the departed down to Hades' realm. The Romans, who called Hades by the name Pluto, identified the helm with the god Orcus, and the poet Virgil in his Aeneid described a similar helmet of darkness worn by the goddess Diana when she wished to move unseen. The idea of a cap that grants invisibility passed from Greek myth into Roman legend and then into the medieval imagination, where it became the tarnkappe of Germanic folklore, the fairy cap that made its wearer invisible and gave him the strength of twelve men.