Helm of Darkness

The Helm of Darkness is one of Hades' symbols of power. According to Grover Underwood, the Helm allows Hades to melt into shadows 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 says that this is why all rational creatures fear the dark.

The Lightning Thief
As the Helm is associated with fear, and Alecto appeared at Yancy right after their former math teacher had a nervous breakdown to get the missing Helm back, it's implied that Hades caused the nervous breakdown. Even without the helm, he still radiates fear. 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 mother, to the world of the living.

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

The Last Olympian
The Helm is shown briefly when Hades entered the second Titan War along 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 and his closely gaurded secerets.

Trivia

 * A helm is shown on the UK book covers of the Percy Jackson and the Olympians series as the logo.
 * Much like how Ares' Sword can appear in different forms, the Helm of Darkness appeared as classic burglurary ski cap. The helm also appeared as a dragons head, a circle of black flames, and wreath of human bones.