Helm of Darkness

The Helm of Darkness, also called the Cap of Invisibility, was given to Hades by the Cyclopes during the Titanomachy. Whoever wore it became completely invisible, even to other gods. It was later lent to Perseus to aid in his quest to slay Medusa, and to Athena during the Trojan War. It represented the power of the unseen, the world of the dead that exists alongside the living, hidden but ever-present.