Helm of Darkness

The Helm of Darkness is one of Hades' Symbols of Power. According to Persephone, 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 most sane creatures fear the dark.
==The Dungeon Master's Theogony==
===Forging the Helm of Darkness===
Zeus' eldest brother, Hades, was very skilled in navigating under the earth and was able to lead all of his siblings into Tartarus, with the intention of freeing the Elder Cyclopes (Arges, Brontes, and Steropes) and Hekatonkeries (Cottus, Gyges, and Briares). All six of them were imprisoned in the maximum-security zone of Tartarus, surrounded by huge bronze walls, and a lava moat guarded by 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 Cronus' 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.
===The First Titanomachy===
Shortly thereafter, Hades successfully wielded his new mighty weapon during the subsequent 11-year-long war with his father Cronus 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.
==The Dungeon Master's Heroes of Greece==
===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.