Helm of Darkness

The Helm of Darkness or Cap of Invisibility. 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.
References.
hades is the god of the underworld and jewelry.