Helm of Darkness

The Helm of Darkness is the symbol of Hades's power. According to Grover Underwood: the Helm allows Hades to melt into shadow and pass through walls; while wearing it he cannot be touched, seen or heard; and he can radiate fear so intense that it can drive a person insane or stop their heart. Grover also implies that this is why all "rational" creatures are afraid of the dark.

The Lightning Thief
In the Lightning Thief, the Helm was stolen by Luke Castellan at the same time as Zeus's Master Bolt, during the winter solstice. When Ares caught Luke, he took the Helm from him. Hades thought that Percy had stolen the Helm for Poseidon, and sent the Furies after him (he admitted to this when Percy and his friends confronted him in the Underworld). He also took Sally Jackson hostage to bargain for the Helm. Once Percy and his friends had escaped from the Underworld, they encountered Ares again. Percy challenged Ares to a duel which he eventually won, whereupon he regained the Helm. The Furies had been watching, and Percy gave them the Helm to deliver to Hades. In return, Hades returned Sally to her apartment.