Stygian Iron

Stygian Iron, like Celestial Bronze and Imperial Gold, is a magical metal. It is capable of absorbing or sucking the essence of monsters and demigods, and injuring Gods, Titans, and Giants. Unlike Imperial gold and Celestial bronze, it can harm both mortals and monsters. It is shown to scare monsters even more then the other two metals, when seen. It also appears to have significant effect on ghosts, and other creatures from the Underworld.

Description
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, it is believed that only deities from the Underworld and their Demigod children can use it, but other powerful beings have been known to have use Stygian Iron.

One large difference between Celestial Bronze and Stygian Iron is the way they kill monsters (and possibly demigods). Celestial Bronze dispells a monsters' essence and returns it to Tartarus, but Stygian Iron is stated to actually destroy it, absorving its essence, which likely means a Stygian Iron weapon permanently destroys monsters. One notable user of Stygian iron is Nico di Angelo, the son of Hades, in the form of a black short sword. Percy describes Nico's sword as being "black as a nightmare."

The Battle of the Labyrinth
After running away from camp, Nico spent some time in the Underworld and managed to forge 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 person he wanted approached. The first mention of Stygian Iron by name is 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 uses his sword to channel his power and raise an army of the dead who obey his orders.

The Demigod Files
During the short story, The Sword of Hades, Persephone orders a new Symbol of Power to be made for Hades. The symbol is a sword made of Stygian Iron, inlaid with the Keys of Hades, and if completed, Thalia, Nico, and Percy wonder if it would tip the scales of power between the Big Three. It is later revealed that Hades did not know about the weapon's creation, but swears on the River Styx to never use it against the Olympians.

The Last Olympian
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.

The Son of Neptune
While Nico is visiting his sister Hazel at Camp Jupiter, Hazel comments that Nico always has his Stygian Iron sword at his side. However, his sword is never used before Nico leaves to find information on the Doors of Death, eventually leading to his capture by Gaea.

Abilities
Stygian Iron is much more dangerous than Imperial Gold or Celestial Bronze. While still able to harm monsters and immortals, it can also harm regular humans. It is also different in the way it destroys its targets. While most magical ores turn monster's 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 also makes it dangerous to even 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. He has also shown to be able to open up fissures to the Underworld by plunging his sword into the ground.

Known Stygian Iron Weapons

 * Nico's sword is Stygian Iron.
 * The Sword of Hades is Stygian Iron.
 * Hades' Staff is made of Stygian Iron.
 * The blade of Thanatos' scythe is made of Stygian iron.

Trivia

 * Imperial Gold is Roman, Celestial Bronze is Greek, and Stygian Iron is from the Underworld. It is then possible that the sons and daughters of Pluto (as well as Hades) could use this metal since the Underworld exists in both Greek and Roman mythology.
 * Only Hades and Nico have been stated to have used Stygian Iron so far.
 * When a monster touches it, it will get its essence 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.