When struck with a mutational "gift" from the Ruinous Powers, most Iron Warriors simply cut off the mutated appendage, if possible, and replace it with a mechanical one.The Iron Warriors were the Emperor's finest siege troops and their Primarch, Perturabo, was the equal of Rogal Dorn in the arts of fortifcation and strategy.
While other Primarchs refused point-blank to see their Legions used in such a way, Perturabo acceded, though with ever-poorer grace.
History records little of the machinations Horus must surely have enacted in order to turn the bitter Perturabo to the cause of the Traitors, but whatever the truth, the Iron Warriors turned upon their brothers at Istvaan V, and in so doing sealed their damnation for all time.
At last freed of the constraints that had bound him, Perturabo gave free reign to his most destructive urges, laying waste to world after world in the Warmaster's service.
While one part of the Legion turned Olympia and its surrounding star systems into an empire of iron, another lent its expertise to the Battle of Terra, and at last, the sons of Perturabo could test themselves against those of Rogal Dorn.
It cannot be known whether Dorn's masterfully constructed defences would have proved the undoing of the Iron Warriors, for the Warmaster was slain before the matter could be fully determined.
Perturabo grew up upon the world of Olympia, a mountainous planet divided into constantly warring city-states.