Helm of Darkness

The Helm of Darkness is Hades' symbol of 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 and animals fear the dark

The Lightning Thief
In The Lightning Thief, the Helm was stolen by Luke at the same time as Zeus' 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 Percy's mother 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 Jackson, Percy's loving mother to the world of the living.

The Demigod Files
The helm is mentioned during the story The Sword of Hades.

The Last Olympian
Here it is shown briefly when Hades entered the battle with Nico, Demeter, Persephone, and Hades' army of dead soldiers. The only thing keeping Kronos' Army from retreating was Kronos himself. They were more afraid of Kronos than Hades and his Helm of Darkness. Percy at one point looks at the helm, and feels as if it was reaching into the darkest corner of his mind and pulling out the things he was most afraid of.

Trivia

 * The helm is shown on the cover of every UK book as the logo.Percy-logo.jpg