A Contest of Nisse and Release
Soulsteel is made from ghosts and the substance of the Labyrinth of the Underworld. It is jet black, and agonized faces of the souls it contains can be seen moving and screaming in the metal. Unsurprisingly, this material is used almost exclusively by the Abyssal Exalted. Soulsteel weapons inflict heavy damage and frequently drain small quantities of Essence as well.
Originally Soulsteel was a fusion of the souls from a forgotten race that rebelled against the primordials, with the metal of the Maker’s own body. This form of the metal which predates humanity, can now only be found in Autochthonia. The Neverborn possibly used Autochthon’s blueprints to make their own version of Soulsteel from human souls.