Dark-Iron

While dwarves are known to produce beautiful artifice when it comes to metalworking. Mithril is the most famous material in dwarven smithing. The darker cousins of the dwarves are also known to create mithril's twin dark-iron. Though synthesized and inferior in many ways like everything Duergar creates it is utilitarian, cruel, and undeniably effective.
Dark Iron has a peculiar quality of never rusting, almost never breaking, and being very accepting of various modifications from the infamous Fire-Pikes to more subtle modifications such as possession or poisons.

Access & Availability

It is a common material, common to the point that it is rarer to see common steel among the Duergar armories than Dark-Iron.

Complexity

Basic, though requires immense heat often relying on magma forges to manufacture.