Stygian Iron

Stygian Iron, like Celestial Bronze and Imperial Gold, is a magical metal capable of killing monsters, demigods and injuring gods, Titans, and Giants. 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, but other powerful beings have been known to have use Stygian Iron, but hardly ever.

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, the son of Hades, in the form of a black short sword. Percy describes Nico's sword as being "black as a nightmare."

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.

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.