When seen, a Stygian Iron weapon is shown to scare monsters even more than a Celestial Bronze or Imperial Gold weapon, possibly because their essence is absorbed into the blade, which possibly prevents the monster from going back to Tartarus to reform. It also appears to have a significant effect on ghosts and creatures from the Underworld.
Stygian Iron is mined and forged in the Underworld and cooled in the River Styx, producing a clean, indestructible blade. Because it is cooled in the River Styx, only children of Hades/Pluto and children of Underworld deities are capable of wielding it, with the exception of some powerful deities who can also wield Stygian Iron.
One large difference between Celestial Bronze and Stygian Iron is the way they kill monsters. Celestial Bronze (as well as Imperial Gold) destroys the monster's physical form and their essence returns to Tartarus, but Stygian Iron is stated to actually absorb/destroy the monster's essence, which most likely means a Stygian Iron weapon can prevent a monster from reforming. One notable user of Stygian Iron is Nico di Angelo, the son of Hades, in the form of a short black sword. Percy describes Nico's Sword as being "black as a nightmare."
The forging process for Stygian iron is similar to that of Stygian Ice, because both are dipped into the River Styx.
After running away from camp, Nico spent some time in the Underworld and managed to obtain a blade of Stygian iron. He first used the sword when Percy saw him in a dream. While Percy did not know what the sword was made of, Nico used the blade to keep the shades of the Underworld away from his sacrifice until the spirit he wanted approached. The first mention of Stygian iron by name was when Geryon captured Nico, and then picked up Nico's sword, careful not to touch the blade. Later, during the Battle of the Labyrinth, Nico used his sword to channel his power and raise an army of the dead who obey his orders, but in doing so exhausted himself and nearly fainted.
Persephone ordered a new Symbol of Power to be made for Hades. The symbol was a sword made of Stygian iron, inlaid with the Keys of Hades. Thalia, Nico, and Percy wondered if it would tip the scales of power between the Big Three. It was later revealed that Hades did not know about the weapon's creation, but swore on the River Styx to never use it against the Olympians.
Hades, when rising from the Underworld, drew his sword to attack the forces of Kronos. His sword was a double-edged Stygian iron blade etched with silver, with one of the Keys of Hades concealed within the sword, which means that it could either raise a soul from Tartarus, or send another soul straight to the Underworld.
While Nico was visiting his half-sister Hazel Levesque at Camp Jupiter, Hazel commented that Nico always had his Stygian iron sword at his side. However, his sword was never used before Nico leaves to find information on the Doors of Death, eventually leading to his capture by Gaea.
While Nico was trapped in the jar, he had his Stygian Iron sword at his side. Later on, the sword is placed somewhere else, in order to lure Hazel Levesque, Leo Valdez, and Frank Zhang into a trap made by Gaea, when Hazel tried to find Nico by detecting the metal used to make his sword. It was later returned to Nico after he was rescued, and the emaciated demigod initially leaned on it like a crutch.
Nico kept his fearsome sword, and used it to slay several katoblepones in Venice, to spar undead warriors from time to time, and later during the climactic battle at the Necromanteion to fight Gaea's army. While helping the Seven fight Clytius, Nico used his sword to dissipate and absorb the giant's dark aura, keeping it from affecting any of his friends again.
Nico took his sword with him during his journey back to Camp Half-Blood with Reyna and Gleeson Hedge. He successfully used it against Gaea's undead Roman army in Pompeii, slashed the bowstring on Orion's bow, took down Octavian's elite guard with it, and subsequently wields the sword during the final battle against Gaea's army in Camp Half-Blood.
Stygian iron is much more dangerous than Imperial Gold or Celestial Bronze. While still able to harm monsters, demigods, and immortals, it can also harm mortals. It is also different in the way it destroys its targets. While most magical metals turn monsters to dust, Stygian iron will literally drain the life force of its target until there is nothing left and it is absorbed into the blade. This makes it dangerous even to those that are already dead, such as ghosts and shades. Stygian iron also has the added benefit of being able to channel the power of the Underworld. For example, Nico has been shown to use his sword to draw the dead to him in order to attack enemies, as well as to be able to open up fissures to the Underworld by plunging his sword into the ground, although this is not likely to have anything to do with the actual blade material.
Known Stygian Iron Items
- Nico's sword
- Hades' sword
- Hades' staff
- Hades' chariot
- Thanatos' scythe
- Clytius' sword
- Clytius' armor
- Koios' armor
- Krios' armor (Possibly)
- Mimas' armor (Possibly)
- Orion's arrow tips
- Nyx's chariot
- The outer frame of the Doors of Death
- Stygian Iron is one of four magical ores, the others being Imperial Gold, Celestial Bronze, and Bone steel.
- Imperial Gold is Roman, Celestial Bronze is Greek, and Stygian Iron is from the Underworld. It is then possible that the children of Pluto (as well as Hades) could use this metal, since the Underworld exists in both Greek and Roman mythology, though this has yet to be confirmed.
- Only Hades, Thanatos, Orion, Tartarus, Clytius, Nyx, and Nico have been stated to have used Stygian Iron so far, making its appearance far more rare than Celestial Bronze or Imperial Gold.
- When a monster touches it, its essence will be sucked into the blade. However, it is unknown if the monster will ever reform or if it will just take longer for them to reform in Tartarus.
- As it is not silver, it probably does not harm lycanthropes.
- Like other enchanted metals, Stygian Iron gives off a faint light; in this case the light is dark purple.
- According to Jason Grace in The House of Hades, Stygian Iron seems to "make shadows even gloomier", as if it "[soaks] the light and heat out of the air."