Infernal Steel is a mysterious, near-impervious metal seen in the hands of Oku during the fall of Steelhold.
Infernal Steel is known to be almost impervious to normal weapons, even those made of high-quality steel. According to Oku, it is supposedly immune to any non-magical weapon, but Emdief pierced the armor with an arrow, at least partially refuting this claim.
The origin of infernal steel is unknown, though its name suggests that it originates from Hell or the True Gods. It has only ever been seen in the hands of Oku Constructcudgel, so whether or not it is more prolific is unknown. Oddly, infernal steel is never mentioned in the Codex Arcana, leading to many doubting its existance.