In the cryptic account of the history of Mankind given by Cripias, one of the Keepers of the Library Sanctus of Terra, the Iron Men or Men of Iron 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 23rd Millennium known as the "Cybernetic Conflict" that extinguished countless lives and destroyed the ancient human-settled galaxy's economic and political unity. 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
The effects of the Men of Iron's rebellion against humanity lasted into the current Age of the Imperium, where the use of artificial general intelligence is banned and seen as abominable and heretical by both the Imperium and the Adeptus Mechanicus. However, Servitors and their myriad variations (combat variants, heavy-lifters, technical assistants, etc.) are commonly used throughout the Imperium with the sanctioning of the Mechanicus' Tech-priests and indeed, Servitors were first developed to replace the Men of Iron. 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, known to the Mechanicus as "Abominable Intelligence."
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.