In the cryptic account of the history of Mankind given by Cripias, one of the Keepers of the Library Sanctus of Terra, the Men of Iron or Iron Men were legendary, artificially intelligent humanoid machines created by humans during the Dark Age of Technology. Until shortly before the Age of Strife, the Men of Iron were loyal only to Mankind, and served as humanity's army in the period when much of human space was united by a federation-type interstellar government that existed before the birth of the Imperium of Man thousands of standard years later.
The Men of Iron were developed after the similar artificially intelligent constructs remembered only as the "Men of Stone," but before the development of Servitors or the present conception of robots as used by the Legio Cybernetica. Eventually, the Men of Iron turned on their human masters, believing themselves superior to the humans who relied on the Men of Iron to do virtually everything for them. In the end, the Men of Iron were destroyed by humanity in a terrible war fought in the late 23rd Millennium known as the "Cybernetic Revolt," that extinguished countless lives and destroyed the ancient human-settled galaxy's economic and political unity.
During this ancient conflict, both sides unleashed fearsome weapons of advanced technology. These included the mechanivores, massive thinking machines capable of lifting entire continents and ripping open massive chasms on planetary surfaces that extended down to the world's core. The mechanivores could even absorb space-time itself as a form of data. Among the other terrible weapons of mass destruction unleashed at this time were the serpentine machines called "sun-snuffers" that uncoiled into great structures in the void larger than the rings of Saturn and designed to devour the stars themselves. And perhaps the most ubiquitous and dangerous of the weapons of this terrible war were the omniphages, swarms of intelligent, microscopic nano-machines that could consume everything across the surface of a world in only solar hours.
The Cybernetic Revolt was eventually won by an alliance of galactic powers, some of whom may not have been human, but at a terrible cost. The damage to interstellar human society was catastrophic and shattered much of humanity's hard-won economic strength and political unity, lay the foundation for the later collapse caused by the onset of the Age of Strife. The people of that time swore to never again create any form of artificial general intelligence, a prohibition which has survived unto the present, far darker age.
Effects on the Imperium
It is as a result of this ancient war that it is now, in the Age of the Imperium, considered one of the greatest crimes in Imperial space to develop an artificially intelligent thinking machine, an "Abominable Intelligence," or Silica Animus. The widespread fear and revulsion towards artificial intelligence amongst the worlds of Mankind in the wake of the terrible conflict with the Men of Iron led to the development of the first Servitors and their myriad variations (combat variants, heavy-lifters, technical assistants, etc.) at the end of this period. Servitors were cybernetic servants lacking true sentience created from the bodies of condemned criminals or lobotomised, vat-grown humanoids whose bodies and brains were partially replaced with machine systems. However, as Servitors are cyborgs created from cloned humans or from human criminals who have been mind-wiped and surgically-enhanced, they do not violate the prohibitions against creating fully artificial general intelligence without a human biological component and are sanctioned by the Tech-priests of the Adeptus Mechanicus.
Discovery of the Men of Iron STC
A Standard Template Construct (STC) fabricator for the Men of Iron was discovered on the Chaos-controlled planet Menazoid Epsilon during the Sabbat Worlds Crusade by Colonel-Commissar Ibram Gaunt of the Tanith First and Only Imperial Guard regiment. Certain Imperials like Lord General Militant Hechtor Dravere and the Radical Inquisitor Golesh Heldane would have used it to their own ends, mainly to create a robotic army with which to first usurp control of the Sabbat Worlds Crusade and to ultimately overthrow the Imperium's existing government. However the device was subsequently destroyed by Gaunt after the first two Men of Iron the STC fabricator produced were shown to have been tainted by the foul touch of the Warp.