Helm of Hades

In classical mythology, the Cap of Invisibility (Ἅϊδος κυνέη (H)aïdos kyneē in Greek, lit. dog-skin of Hades) is a helmet or cap that can turn the wearer invisible, 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. Those wearing the Cap become invisible to other supernatural entities, akin to a cloud of mist sometimes used to remain 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 Ouranian Cyclops give Zeus the lightning bolt, Poseidon the trident, and a helmet (kyneê) to Hades in their war against the Titans.