Stygian Iron

Stygian iron, like celestial bronze and Imperial Gold, is a metal capable of killing monsters. However, Stygian iron is much rarer and unlike Imperial Gold and Celestial Bronze it can harm both mortals and monsters. Stygian iron is forged in the Underworld and cooled in the River Styx, producing a keen, indestructible blade. Because it is cooled in the River Styx, only Hades and his children, God or demigod may possess it.

One large difference between celestial bronze and Stygian iron is the way they kill monsters. Celestial bronze turns the monsters to dust, but Stygian iron absorbs their essence until they are killed (once killed they cannot reform). One notable user of Stygian iron is Nico di Angelo, in the form of a black short sword. In The Last Olympian, Hades has a double-edged Stygian iron blade etched with silver. Percy describes it as "black as a nightmare".

Trivia

 * Imperial Gold is Roman, Celestial Bronze is Greek, and Stygian Iron is from the Underworld.
 * Only Hades and Nico have been stated to have used the Stygian Iron(hinting that only Hades and his children could use it).
 * Children of Hades can summon Stygian iron.
 * When a monster touches it, it will avoporate into the blade causing it to be more powerful, (and possibly regovanate the owner).
 * When a demigod, god or monster touches it, their body will start to freeze.