The Helm of Darkness was the main weapon of Hades. It is also known as The Helm of Hades or the Cap of Invisibility. Hades has been known to lend the Helm off to other gods, and once to a demigod.
Myth
Creation
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.
Perseus
Perseus was a demigod who the gods favoured. He received temporary gifts that would help him slay Medusa. From Hermes, he received Winged Sandals, from Athena, he received either a shield made of mirror or a bronze shield that was polished so it would reflect, a sword from Hephaestus, and the Helm of Darkness, from Hades. Perseus would use this to sneak into the cave where Medusa lived. He would later return it, after rescuing his mother.
Hermes
Hermes borrowed and used the Helm of Invisibility in a battle with Hyppolitos, the giant.
Trojan War
Athena borrows and uses the Helm of Invisibility to help Diomedes. She used it to sneak past Ares, as Diomedes and Ares were enemies.
Powers and Abilities
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.