Helm of Darkness

The Helm of Darkness or Cap of Invisibility.Ἄϊδος κυνέην (H)aidos kuneēn in Greek, meaning dog-skin of Hades in Greek mythology, is a helmet that makes the person wearing it invisible. It is also called the Cap of Hades or Helm of Hades. It was said to be used by the goddess of wisdom, Athena, the messenger god, Hermes, and Perseus. 

According to mythology, it was made by the Cyclops. They had made a lightning bolt for Zeus, a trident for Poseidon, and the Helm of Darkness for Hades.Grimal, Pierre. The Dictionary of Classical Mythology. Trans. A. R. Maxwell-Hyslop. Blackwell Publish, 1996.
The helm of darkness is also mentioned in Percy Jackson.
 References 

Category:Greek mythology