The Steel Brethren, also known as the Steel Brotherhood, are a warband of Chaos Space Marines drawn from the Iron Warriors Traitor Legion. Following their parent legion's path, they are dedicated to Chaos Undivided. They have been noted to make extensive use of gunship transports and Drop Pods in their planetary assaults. The Steel Brethren, like all warbands of the Iron Warriors, possess extraordinary skill and experience in siege warfare. The Steel Brethren, when away from the Iron Warriors' Daemon World of Medrengard, operate from the Chaos Cruiser identified as the Ferrum Invictus.
- The Dispute of Iron (c.600-730.M34) - Its cause hidden to the eyes of the Imperium, a vast civil war erupts on the Daemon World of Medrengard and rapidly spreads to the other domains of the Iron Warriors. Ancient pacts of fealty and alliance are called upon and scores of Chaos Space Marine warbands, Daemons and Traitor Titan Legions are drawn into the maelstrom of relentless battle. This bitter feud is accredited at creating several sub-factions of Iron Warriors still active to the present day, such as the Steel Brethren, while utterly destroying others such as the Shattered Tower. As abruptly as it was begun the war suddenly ended, leading some to believe that the official conflict had been carried out according to the Daemon-Primarch Perturabo's design in order to weed out the weak and the unworthy from his scions.
- The Siege of Vraks (813-830.M41) - The Steel Brethren were one of the 11 warbands of Chaos Space Marines who heeded the Alpha Legion Chaos Lord Arkos the Faithless' call to the Imperial Armoury World of Vraks Prime to take part in the slaughter and mayhem that marked the 17-year-long Siege of Vraks by Imperial forces. Their experience in counter-siege warfare made for a powerful addition to the Traitors’ forces, most notoriously holding back two attacks in the tank ditch protecting the entry into Vrak’s large underground storage bunkers prior to the fall of the citadel. Their extensive knowledge of techno-arcana is believed to be the reason for the (re)appearance of several of the venerable and powerful Valdor Tank Hunters which had until then laid slumbering in Vraks’ armouries. The warband is believed to have claimed several of these mighty vehicles for their own needs and perhaps having even shipped some models offworld. It is however unclear if the Steel Brethren proceeded to flee alongside their Khornate allies through the Immaterium or were trapped and subsequently destroyed with the fall of the citadel as the Ferrum Invictus was not seen in the Vraks-system before the end of the Siege.
The Steel Brethren wear primarily metallic, steel-coloured Power Armour, with black on the inset of the shoulder plates, essentially a minor variation of the standard colour scheme employed by the Iron Warriors.
The Steel Brethren's warband badge is two red Iron Halos that are interlinked on a black background.
- Codex: Chaos Space Marines (4th Edition), pg. 24
- Imperial Armour Volume Six - The Siege of Vraks - Part Two
- Imperial Armour Volume Seven - The Siege of Vraks - Part Three, pp. 137, 172
- Imperial Armour Volume Thirteen - War Machines of the Lost & The Damned by Andy Hoare with additional material by Alan Bligh & Neil Wylie, pp. 22, 30, 51