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.
As the Helm is associated with fear, and Alecto appeared at Yancy Academy 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 Castellan 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 Jackson 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 Helm is mentioned during the story The Sword of Hades.
The Helm is shown when Hades entered the second Titan War along with Nico di Angelo, 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. It can be noted that Nico did not seem affected by his father's helm, even though he is not a god, suggesting that the children of Hades could be immune to its effects or experience them at a smaller degree. 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 guarded secrets. He states that he 'wanted to crawl into a hole and hide'.