Dark Age of Technology - The Men of Iron

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 government that existed before the Imperium of Man. The Men of Iron were developed after the similar artificially intelligent constructs remembered only as the Men of Stone, but before the "modern" conception of robots in the Warhammer 40,000 universe. 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 that extinguished countless lives and destroyed the ancient galaxy's economic and political unity. The people of that time swore to never again create advanced artificial intelligences, a prohibition which has survived unto the present, far darker age.

The effects of the Men of Iron's rebellion against humanity lasted into the current Age of the Imperium, where the use of artificial intelligence is banned and seen as abominable and heretical by both the Imperium and the Adeptus Mechanicus. However, Servitors and their 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 truly artificial intelligences.

A Standard Template Construction (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 would have used it to their own ends, mainly to create a robotic army with which to overthrow the Imperium, but it had been tainted by Chaos and was subsequently destroyed by Gaunt after the first two Men of Iron the STC fabricator produced were tainted by the foul touch of Chaos.This in itself may be evidence of what happened to the original Men of Iron. As sentient beings, they could have been corrupted by Chaos and then turned on their creators.

http://warhammer40k.wikia.com/wiki/Men_of_Iron