The Iron Circle was the personal bodyguard of Perturabo, Primarch of the Iron Warriors, a phalanx of Domitar-pattern battle-automata forged to his own exacting design. Where other Primarchs surrounded themselves with the finest warriors of their Legions, the master of siege-craft chose instead the cold loyalty of machines, robots that could never waver, never betray, and never question his will.
Each automaton of the Iron Circle bore a great slab-shield and a crushing weapon, forming an impenetrable ring of adamantium around their maker. Governed by cortex controllers rather than living minds, they answered only to Perturabo, embodying his conviction that flesh was weak and treacherous where iron was not.
The Iron Warriors understood the bitter message written in their gene-father's guard. Having endured decimation and mistrust, Perturabo had come to place more faith in unfeeling steel than in the sons who marched at his side.