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 easily drive a person insane or stop their heart. Grover also says that this is why all rational creatures fear the dark.
Forging the Helm of Darkness
Zeus' eldest brother, Hades, was very skilled in navigating under the earth, was able to lead all of his siblings into Tartarus, with the intention of freeing the Elder Cyclopes (Arges, Brontes, and Steropes) and Hekatonkheires (Cottus, Gyges, and Briares). All six of them were imprizoned in the maximum-security zone of Tartarus, surrounded by huge bronze walls, and a lava moat, guarded fierce demons. Their guardian, Kampê, was the most ferocious and fearsome monster in all of Tartarus, and even Zeus, Poseidon, and Hades initially shuddered with horror when they saw the infernal monster for the first time. However, the gods overcame their fear, and were able to sneak in. Zeus managed to talk to the Cyclopes Brontes, and convinced him to forge powerful weapons for him and his siblings (which would be more powerful than Kronos' Scythe) behind Kampê's back. Together with his brothers Steropes and Arges, Brontes forged three incredibly powerful weapons, the third of which was the Helm of Darkness, which was given to Hades. The helm instantly made him invisible, and generated such ineffable terror, that it scared his brothers Zeus and Poseidon to the point that both of them paled and started to sweat with fear, greatly satisfying Hades.
Shortly thereafter, Hades successfully wielded his new mighty weapon during the subsequent 11-year-long war with his father Kronos and the other Titans. The Titans initially had the upper hand in the battles, since they were very skilled, and much more experienced warriors. Hades, however, quickly became a skilled, dangerous, and ferocious warrior as well, and greatly contributed to the Titans' ultimate defeat in the war, with his terrifying Helm earning him the fear of all of the Titans.
Hades' Symbol of Power
After the Titanomachy, the gods (who now called themselves the Olympians) chose Mount Olympus as their official residence, and Zeus as their King. Hades would receive the Underworld, and while his terrifying Helm would remain his personal weapon in times of need, it also became his symbol of power, along with his two-pronged staff, and his three-headed pet dog Cerberus. Due to how terrifying his helm was, however, Hades instantly became so feared by mortals and demigods, that they rarely even used his name, and instead referred to Hades as either "The Rich One", "The Silent One", or "The Hospitable One".
Hades was later seen wearing the Helm of Darkness while invisibly spying on his beloved Persephone, and later when revealing himself before kidnapping her.
Perseus' Replicated Helm
Right before the demigod Perseus departs on his quest to find the Gray Sisters (and, subsequently, slay Medusa), he is visited by Hermes, who gifts him with a magical expandable sack, winged sandals, and a replica of Hades' Helm of Darkness. While it was far less powerful than the actual helm (in that it didn't make Perseus inaudible, intangible, and capable of dissolving into shadows), the replica did successfully turn him invisible.
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, so it is implied that Hades caused the nervous breakdown, as even without the helm, he still radiates incredible infernal terror. 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 going with Nico di Angelo, Demeter, Persephone, and his army of dead soldiers. The only thing keeping Kronos' Army from retreating was Kronos's authority. It can be noted that Nico did not seem affected by his father's helm, suggesting that the children of Hades could be immune to its effects or experience them to a much smaller degree. Percy at one point looks at the Helm, and even though the Helm's power wasn't directed at him, he still felt 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, making Percy want to "crawl into a hole and hide".
Hades participates in the final battle of the Second Giant War in Athens, though he isn't seen, due to him being invisible while wearing the Helm of Darkness. The combined efforts of Hades, Hazel and Arion overpower Alcyoneus, after which the Lord of the Dead sends the fallen Giants' bodies back to Tartarus by opening up abysses under them right after each Giant is killed.
The fearsome Helm was forged out of Celestial Bronze (decorated with scenes of death and destruction), in Tartarus by the Elder Cyclopes, when the children of Kronos came to rescue them. It was the last of the three most powerful weapons ever forged for the Olympians, with it being made right after Zeus' Master Bolt and Poseidon's Trident, the only 2 weapons that rival it in power. While the Helm did not initially seem to be a standard offensive weapon to Hades, he soon came to appreciate the great psychological and strategical power offered by it. As a result, the Helm of Darkness grants the Lord of the Dead several formidable powers:
- Imperceptibility: while wearing it, Hades can become invisible, similar to Annabeth's Yankees Cap and Perseus' replica helm, though the Helm is described as much more powerful, since it allows him to become a shadow. This means that while wearing it, Hades can pass through walls and melt into shadows, not to be touched, seen or heard by anyone, even other Olympians, Titans, and Giants.
- Induced Terror: with it, Hades can radiate death and terror to a tremendous degree. In The Last Olympian, Kronos' entire army tried to flee from him in terror, and only Kronos's authority allowed them to somewhat keep their ranks. Even though the Helm's power wasn't directed at him, when Percy looked at it, he still felt 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, making him want to "crawl into a hole and hide". Grover claims that when utilized at full power, the Helm's aura of ineffable deadly terror is so intense, that it can easily unhinge people's minds and stop their heartbeats, which, according to Grover, is why most rational beings fear the dark. In Percy Jackson's Greek Gods, Hades' Helm is shown to be powerful enough to scare even Zeus and Poseidon simultaneously, to the point that both of them paled and started to sweat with fear. It should be noted that Nico did not seem affected by his father's Helm, suggesting that the children of Hades could be immune to its effects or experience them to a much smaller degree, though it might have also been because the Helm's power wasn't directed at him at the time.
- Much like how Ares' Sword can appear in different forms, the Helm of Darkness appeared as classic ski cap, a dragon's head, a circle of black flames, and a wreath of human bones in The Lightning Thief. As seen in Percy Jackson's Greek Gods, however, the Helm's actual form is one of a gleaming Celestial Bronze Greek war helmet, decorated with scenes of fearsome death and destruction.