From Lexicanum

In the cryptic account of the ages of Mankind given by Cripias, one of the Keepers of the Library Sanctus of Terra, the Men of Iron were legendary sentient 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 their army. They came after the Men of Stone, but before the "modern" conception of robots in the Warhammer 40k universe. Eventually, they 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.

Effects on the Imperium

The effects of this last into the current Age of the Imperium, where the use of artificial intelligence is banned and seen as abominable. However, servitors and their variations (combat variants, heavy-lifters, technical assistants, etc) are commonly used throughout the Imperium with the sanctioning of Tech-priests.

Discovery of the STC

A Standard Template Constructor for the Men of Iron was discovered on the planet Menazoid Epsilon during the Sabbat Crusade by Colonel-Commisar Ibram Gaunt of the Tanith First and Only. It had been tainted by Chaos and was subsequently destroyed by Gaunt.

Sources

• Warhammer 40,000 3rd Edition Rulebook, The Journal of Keeper Cripias
• First and Only by Dan Abnett