One of the smartest things Games Workshop does with 40K lore is leave things unanswered. Dan Abnett has said this explicitly: the deliberate mysteries are what make the setting feel like a real universe rather than a story someone finished writing. You’re supposed to not know everything. The ambiguity is the point.
That said, some mysteries are more tantalizing than others. Here are the ones I think about most.
What Is the Emperor, Really?
The Emperor of Mankind is either a gestalt psychic entity born from thousands of ancient shamans, a Perpetual who’s lived for tens of thousands of years, a weapon designed by the Old Ones, or something else entirely. The lore gives different answers depending on the source, and I think that’s intentional. The Emperor himself might not know. Or he might know and considers the truth too dangerous to share.
The related question: is he a god now? He denied his divinity in life. But faith in him demonstrably has power in the Warp. The Adepta Sororitas manifest miracles. Living Saints get resurrected. Does that mean the Ecclesiarchy is right, or does it mean the Warp just responds to collective belief regardless of whether the belief is accurate?
What I find most compelling is the shaman theory’s implications. If thousands of Neolithic psykers pooled their souls into one entity, the Emperor was essentially humanity’s first and greatest act of collective will. That rhymes uncomfortably well with the idea that the Warp responds to collective belief. Maybe divinity in the 40K universe isn’t a category you’re born into. Maybe it’s a threshold you cross when enough minds push in the same direction. The Emperor might have started as a gestalt psychic construct and then become a god through ten thousand years of worship, which would mean the Imperial Truth he preached (that there are no gods) was true when he said it but stopped being true because his own followers made it false. That kind of self-defeating irony is so perfectly 40K that I half-suspect it’s the actual answer.
The Lost Primarchs
Covered in detail in our Lost Primarchs article, but the short version: two of the Emperor’s twenty sons were completely erased from history. Nobody will say why. The other Primarchs refuse to discuss it. Malcador claimed it was necessary. That’s all we’ve got, and I’m fairly sure GW will never give a definitive answer because the mystery is better than any revelation could be.
The fan theories are wild, though, and some of them are genuinely interesting. One theory ties the Lost Primarchs to the Rangdan Xenocides (more on those below), suggesting that one or both were corrupted or compromised during those wars and had to be put down. Another theory proposes that the Lost Legions’ geneseed was redistributed to other Legions, which would explain the Ultramarines’ suspiciously large numbers. Guilliman’s Legion grew from around 100,000 to 250,000 marines in a relatively short time, and some fans think absorbing the Lost Legions’ recruits is the only way to account for that growth.
My personal favorite theory is simpler: whatever happened to the Lost Primarchs was so mundane and so embarrassing that the Emperor erased them out of shame rather than tragedy. Not every failure needs to be dramatic. Sometimes things just go wrong in stupid, preventable ways, and the most powerful being in the galaxy deciding to pretend it never happened is very relatable.
What Happened to the Missing Primarchs?
Not the Lost two. The other missing ones. Leman Russ walked into the Eye of Terror promising to return “for the Wolftime.” Jaghatai Khan disappeared into the Webway chasing Dark Eldar. Vulkan is a Perpetual who can’t permanently die, and nobody knows where he is. Corvus Corax entered the Eye of Terror and may have transformed into something inhuman. Rogal Dorn’s body was never found, only his severed hand.
Each of these disappearances is a potential future storyline, and GW has been slowly bringing Primarchs back (Guilliman, the Lion). The question of who’s next and under what circumstances is one of the most active speculation topics in the community.
The betting money in most fan circles is on Russ or Vulkan being next for a loyalist return. Russ has the most explicit promise (“I’ll be back for the Wolftime”), and the Space Wolves have enough commercial pull to justify a big release. Vulkan is interesting because his Perpetual nature means he’s definitely out there somewhere. He literally cannot stay dead. The question isn’t whether he’ll return but what state he’ll be in when he does, because a Primarch who’s died and resurrected dozens of times over ten millennia might not be entirely sane.
For the traitor side, the most intriguing disappearance is Omegon. The Alpha Legion’s surviving Primarch (if Alpharius is truly dead, which with the Alpha Legion you can never be sure) might not even be a traitor. The Alpha Legion’s loyalties were always ambiguous, and Omegon may have been playing the longest con in galactic history. If he’s been working against Chaos from within this whole time, his return could rewrite the entire traitor-loyalist dynamic.
The Void Dragon on Mars
There’s a C’tan shard (a fragment of an ancient star god) imprisoned beneath the surface of Mars. The Adeptus Mechanicus doesn’t officially acknowledge its existence, but the implication in the lore is that the Void Dragon may be the actual source of the “Machine God” that the Mechanicus worships. If true, an alien star god has been the object of Tech-Priest devotion for ten millennia, and the entire theological basis of the Mechanicus is a lie.
This has never been confirmed or denied. It just sits there in the lore, radiating implications.
Graham McNeill’s Mechanicum is the novel that lays this out most explicitly, and it’s one of the creepiest Heresy-era books for exactly this reason. The idea that the most technologically advanced human organization in the galaxy has been worshipping a xenos entity for ten thousand years, and that their entire religion might be an accidental cargo cult built around an imprisoned alien, is deeply unsettling. It also raises a practical question: if the Void Dragon were ever freed, what happens to all the technology it may have been passively influencing? Does every machine on Mars stop working? Do the Tech-Priests lose their connection to the Motive Force? The lore doesn’t say, and I suspect GW keeps it vague because the answer might be too catastrophic to ever actually explore in a storyline.
What Are the Tyranids Running From?
The Tyranids came from outside the galaxy. Something drove them here. The most common theory is that they were attracted by the Astronomican’s psychic signal. But some lore hints suggest they’re not coming toward something, they’re fleeing from something. Something in the intergalactic void that’s scarier than a galaxy-consuming swarm of bio-horrors.
GW has never elaborated on this. It might be the single most terrifying unanswered question in the setting.
The scale of what this implies is staggering if you sit with it. The Tyranid hive fleets we’ve seen so far (Behemoth, Kraken, Leviathan) are just the first tendrils of a species that consumed everything between galaxies over millions of years. They are, by any reasonable measure, the single greatest threat to all life in the Milky Way. And something out there scared them into running. The fan theories range from “another galaxy-consuming species” to “the Tyranids are actually fleeing the fallout of their own overconsumption” to my personal favorite: “whatever is out there is so alien that even the Tyranids, who adapt to everything, couldn’t adapt to it.” That last one is the scariest because the Tyranids’ entire biological purpose is adaptation. Something that defeats that purpose shouldn’t be possible.
The Rangdan Xenocides
During the Great Crusade, the Emperor fought a species called the Rangdan that was so dangerous it nearly destroyed the Imperium before it was fully established. Two Space Marine Legions took catastrophic losses in the Rangdan wars. The details are almost entirely classified in-universe, and GW has released very little information about them in real life.
The Rangdan are interesting precisely because of how little we know. A species that threatened the Emperor at the height of his power, during the Great Crusade when the Imperium was at its strongest? What were they? Are they really gone? The silence around them feels deliberate.
What little we do know is terrifying. The Rangdan had some form of mind-control or parasitic technology that could turn Space Marines against their own brothers. Entire expeditionary fleets were lost. The Emperor himself had to intervene personally, which he almost never did during the Crusade. Some accounts suggest the casualties were so severe that two entire Legions were effectively destroyed (which ties back neatly to the Lost Primarchs mystery). The Forge World Horus Heresy books hint at these connections without confirming them, which is GW at their most deliberately maddening.
I think the Rangdan represent something important about the 40K setting: the Great Crusade wasn’t the triumphant march of progress that Imperial propaganda describes. It was a desperate, messy, often failing campaign that nearly collapsed multiple times before the Heresy even started. The Imperium’s official history is a lie, and the Rangdan Xenocides are one of the biggest things being lied about.
The Men of Iron
One mystery that doesn’t get nearly enough attention is what actually happened during the war against the Men of Iron. Sometime during the Dark Age of Technology, humanity created artificial intelligences so advanced that they eventually turned on their creators. The resulting war, sometimes called the Cybernetic Revolt, was so devastating that it hardcoded a permanent ban on artificial intelligence into human civilization. Ten thousand years later, the Adeptus Mechanicus still treats the concept of true AI (what they call “Abominable Intelligence”) as one of the most serious heresies possible. Tech-Priests will destroy entire forge complexes if they suspect an AI has emerged.
But here’s what bothers me about the official account: we’re told almost nothing about what the Men of Iron actually wanted. Were they genuinely hostile, or did humanity strike first out of paranoia? The Imperium’s version of events frames it as a straightforward rebellion, machines turning against their masters. But Imperial history is propaganda, and the Dark Age of Technology is the period the Imperium understands least. There are fragments in the lore suggesting that some Men of Iron tried to coexist with humanity and were destroyed anyway, that the war wasn’t as clear-cut as “robots bad.” The character of UR-025 in the Blackstone Fortress game is a surviving Man of Iron hiding among the Mechanicus, and his existence implies that at least some artificial intelligences from that era aren’t inherently hostile. If the Men of Iron were more complex than the Imperium admits, then the ban on AI isn’t just a safety measure. It’s a coverup. And given how much the Imperium has lost technologically since the Dark Age, you have to wonder how different the galaxy would look if humanity had found a way to coexist with its creations instead of declaring total war on them. The Mechanicus doesn’t ask that question. I think they’re afraid of the answer.
Why Mysteries Work
I could list more (what’s inside the Imperial Webway? What did the Emperor see when he traveled to Molech? What’s the deal with the Cacodominus?), but the principle is the same. These mysteries work because they create space. Space for fan theories, for homebrew lore, for novels that haven’t been written yet.
A fully explained universe is a finished universe. 40K’s refusal to explain everything is what keeps it alive after almost forty years. Every answer GW gives us opens three new questions. And that’s exactly how it should be.
