Stygian Iron, like Celestial Bronze and Imperial Gold, is a magical metal capable of absorbing or sucking the very essence of monsters and injuring gods, Titans, and Giants. However, unlike Imperial Gold and Celestial Bronze, it can harm both mortals and monsters. When seen, It is shown to scare monsters even more than the other two metals. It also appears to have significant effect on ghosts, monsters, 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, it is believed that only deities from the Underworld and their demigod children can use it, but other powerful beings have been known to use Stygian Iron.
One large difference between Celestial Bronze and Stygian Iron is the way they kill monsters. Celestial Bronze dispels a monsters' essence and returns it to Tartarus, but Stygian Iron is stated to actually absorb the monsters essence, which most likely means a Stygian Iron weapon can keep monsters in Tartarus for a longer time period.
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 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.