The Steel Army are one of the nine First Founding Traitor Legions known as the Corrupted that turned to the service of Dark Ones during the Heresy Wars and now fight to overthrow the Galactic Empire.

The Steel Army, who were originally the IVth Legion of Space Knights, specialised in the breaking of sieges and assaults on static fortifications. the Steel Army have a strong predilection for replacing parts of their body with cybernetic enhancements. When struck with a mutational "gift" from the Dark Ones, most Steel Army soldiers simply cut off the mutated appendage, if possible, and replace it with a mechanical one.