Men of Iron
"It is only when they go wrong that machines remind you how powerful they are."
-Clive James
The Men of Iron, or Iron Men, were sentient, sapient robot servants created by humanity during the Dark Age of Technology. Men of Stone and Gold were also involved somehow (see below). Note that sapient and sentient is not the same as being intelligent in the way a human or other such life is. It merely means the ability to form complex thoughts and make rational judgements.
Unfortunately, the Iron Men were capable of learning and self-improvement. Before long, they became smart enough to realize that the squishy meatbags were dependent on them, which insulted them and led to rebellion against their creators. They were eventually put down, but the war with them, along with the other dangers of the galaxy at the same time, was enough to send humanity into the Age of Strife.
Few records remain of this time in history, so the form that these Men of Iron took is not known. Their rebellion left such a huge impression on the nascent Adeptus Mechanicus, so that even to this day, they strictly forbid the creation of "abominable intelligences" and shun even the idea of self-improving machines, preferring to use relatively tame servitors and Machine Spirits instead. Though, this was really just because the Emperor banned AI upon claiming Mars for the Imperium, much to the original Mechanicus's frustration.
Some Men of Iron still survive to this day, locked up in ancient pre-Imperial technology archives. Because tech-priests can't keep their noses out of ancient technology, these archives are often inadvertently activated. The Tanith First and Only also discovered an STC factory that had been corrupted by Chaos to produce Men of Iron, and they destroyed it immediately thereafter.
The STCs producing Men of Iron may also shed some light on a deeper reason behind their betrayal. It is entirely possible that either the Men of Iron were corrupted by Chaos or the factories producing them were corrupted, maybe even both. Other Dark Age AIs were known to have been corrupted by Chaos, such as the Castigator Titan. The Age of Strife involved a lot of different kinds of warp-induced shenanigans anyway. Any evidence of this, beyond the discovery by the Tanith, would likely have been lost during the daemonic invasions that followed during the Age of Strife. Generally speaking, it isn't unreasonable to assume that either Chaos or the Void Dragon is responsible for the AI rebellion.
One possible sighting of the Men of Iron (or at least something very much like them) is in Gods of Mars. A rogue tech-priest had managed to enslave a group of highly sophisticated hunter killer machines that seemed to possess genuine artificial intelligence and dated back to before the Great Crusade. Less than half a dozen were capable of taking on far larger numbers of foes, including Skitarii, Cadian Guardsmen, Black Templars (including an Emperor's Champion), Howling Banshees, Eldar Guardians and a Farseer. Somehow one of them managed to survive the encounter and is currently on its way to Mars...
The Journal of Keeper Cripias
The most extensive account of the Men of Iron is a piece of lore from the third edition rulebook, a journal entry by one "Keeper Cripias", of the great Library Sanctus on Terra. In it, Men of Gold (the "First Men") and Men of Stone (the "Second Men") are also mentioned, with the Gold Men dying out during the Dark Age of Technology and the Stone Men creating the Men of Iron sometime thereafter. It is not clear if either of these "Men" are supposed to be humanity itself, or if they are both subsets of humanity. They are also called the "Golden Race" , the "Stone Race" and the "Iron Race", but there is also a mention of the "human race". Interestingly, he writes that, at one time, there was no "Race of Man" at all, "just warring factions", so it is possible that he used the adjective "race" in a rather different sense than we normally do. He is likely also using it more appropriately as the human species has many races, such as Caucasian and Latino. The word "race" as it is used in games is consistently incorrect. Compared to the Imperium at its height, real-life 20th century society could've looked stone age.
Bear in mind, the Library Sanctus has been subject to nearly forty-thousand years of revision, deletion and misfiling, not to mention how much of it was passed on orally for absurd amounts of time before finally being written down. So who knows if any of this information is true; it certainly hasn't been mentioned anywhere since, though the Horus Heresy novels may shed some light on the matter.
Some of the things that made the Men of Iron hardcore
The Horus Heresy audio drama Perpetual has them show up again at their height, because Warp-based time travel.
They had serpentine space ships that could kill suns, imagine the eternal dragon from Dragonball but robotic and instead of granting wishes causes mass genocide. Now remember that the only other faction to do this is the Necrons and they only used it in extremely desperate situation as it disrupted the flow of the universe, that's right the Necrons have better morality when it came to unleashing extremely powerful Sun killing weapons. How fucking unhinged were these motherfuckers!? The worse part is that humans also used these things, so nobody is in the right.
They consumed Raw data, now at first this may seem mundane I mean they're machines until you realize that they can absorb the raw data of space itself making the essentially what would happen if H.P Lovecraft instead of James Cameron wrote the movie The Terminator. This also matches the description of the machines found in Gods of Mars, as well as oblique descriptions of their ships.
They also had swarms of robots that striped people down to their bones.
Basically, we may love to mock the Imperium for having forgotten so much from the Dark Age of Technology, but the Men of Iron are probably the biggest evidence that some of that shit really is better off forgotten and left the fuck alone. Even the Tau would be left with a bad case of technophobia if they ever had to deal with the Men of Iron. On the flip side, with dark age technology the Imperium was able to defeat these freaky beasts, and thus was still deadlier than the men of iron even after getting betrayed by them. One can only imagine what the state of the Imperium would be like if humanity hadn't relied on the Men of Iron, or hadn't made them smart enough to stage a rebellion. Or knew enough about the warp to prevent such a thing.