Stygian Iron

Stygian Iron, like Celestial Bronze and Imperial Gold, is a magical metal capable of killing monsters, demigods and injuring Gods and Titans. However, Stygian iron is rare, and unlike Imperial gold and Celestial bronze, it can harm both mortals and monsters. Stygian iron is mined and forged in the Underworld and cooled in the River Styx, producing a keen, 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.

One large difference between Celestial bronze and Stygian iron is the way they kill monsters (and maybe demigods). Celestial bronze turns the monsters to dust, but Stygian iron absorbs their essence until they are killed. And after they are killed, it takes a longer time before they 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, with one of the Keys of Hades concealed within the sword which means that it can either raise a soul from Tartarus, or send another soul straight to the Underworld. 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 Stygian Iron so far.
 * When a monster touches it, it will get its soul sucked into the blade.