The Cap of Invisibility (Ἅϊδος κυνέην (H)aïdos kynein in Greek, lit. dog-skin of Hades) is a helmet or cap that can turn the wearer invisible. It is also known as the Cap of Hades or Helm of Hades. Wearers of the cap in Greek myths include Athena, the goddess of wisdom, the messenger god Hermes, and the hero Perseus. 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.

One ancient source that attributes a special helmet to the ruler of the underworld is the Bibliotheca (2nd/1st century BC), in which the Uranian Cyclops give Zeus the lightning bolt, Poseidon the trident, and a helmet (kyneê) to Hades in their war against the Titans.

In classical mythology the helmet is regularly said to belong to the god of the underworld. Rabelais calls it the Helmet of Pluto, and Erasmus the Helmet of Orcus. The helmet becomes proverbial for those who conceal their true nature by a cunning device: "the helmet of Pluto, which maketh the politic man go invisible, is secrecy in the counsel, and celerity in the execution.

The Helm of Darkness was created after the Cyclops were freed during the Titanomachy. They were so grateful for being set free, they created weapons to aid the gods in the war. For Hades, they created the Helm of Darkness. He used it in this war and kept it afterwards.

The Helm of Darkness allows the user to turn invisible. It also allows dominion over the Underworld and the dead, which happened after the Titanomachy. It is said to radiate fear.