Stygian Iron channels solidified hellfire.
Stygian Iron is a material created by the Demons in Avernus as a response to the invention of holy Seraphite. Formed from dark, reinforced iron plunged into vats of seething Hellfire, this metal is particularly wicked when put to use against holy beings and users of Light.
While it is always warm to the touch, in response to holy magic stygian iron burns with ravenous demonic flames. Wielding a blade forged from this material without having any Dark in your blood is like holding white-hot steel in your naked palm.
Those who wield stygian iron specialize in the destruction of Angels. The metal burns them more than any other being, and cuts made by a stygian iron blade into the flesh of an angelic being necrotize rapidly. In an obscene mimic of the Heretic's Plague inflicted by seraphite, the beings poisoned by the touch of stygian iron fall prey to the sickness of Hellfever. Seeing as it is still technically iron despite its unholy forging, stygian iron is also effective and poisonous against the Fae.