"Wouldn't it be cool if..." - What's your headcanon?

[Kaldoth]

Was having a convo with a buddy of mine over the lore in 40k and just how many retcons, loose ends, and open avenues exist for the setting. We got into our favourite pieces of headcanon and it made me wonder what everyone else has going on in their brain. Is there an unresolved piece of the story you've got a theory about, or an ending to a loose thread you think would tie it up nicely?  I wanna hear what you've got!

 

I'll start off with my headcanon about The Khan. So the Khan disappears in the Webway chasing Dark Eldar and we never hear from him again, right? He's the only loyalist Primarch that pulls a disappearing act without a final message to his men. Vulkan left his writings, Corax's "Never more," Russ's Wolftime spiel, etc. Why no message from Khan? Well, that's because Khan didn't mean to disappear. His ending was more akin to that of Guilliman and the Lion, and he's absolutely still alive... In the Black Library.

 

Khan gets into the Webway, gets lost, and eventually is surrounded and captured by the very Dark Eldar he was chasing. His men are slaughtered, and he is taken to Commoragh and made to endure existence as a gladiator in the fighting pits. He is the city's greatest but most secretive attraction. Only the highest of Archons receive the privilege of watching him fight. After enduring torture and endless combat for millennia his mind is broken and his body modified with all manner of sadistic scars and combat modifications; The Emperor's most stoic and taciturn son flesh warped into a horrifying force of murder. 

 

As the ages of brutal combat go on and on, his sanity slowly begins to wane, and his thoughts are replaced with laughter... but the laughter isn't his own. Inside his head, there is another voice that grows louder with each combat, both heightening the madness and yet keeping it at bay, but always taunting him. In the most violent combat he is forced to endure, he manages a breakout from the arena and tears through Commoragh, chasing after the voice in his own head. He eventually escapes on a jet bike and launches himself back into the Webway to chase the laughter. That laughter leads him to the Harlequins, who then subdue him and take him to the Black Library. 

 

Upon arriving, the Black Council restrain him and psychically repair the damage to his body and mind. When he wakes, the laughter has finally stopped, but it has been replaced with all of the knowledge of Chaos, the Black Crusades, all that has come to pass within the Imperium, and the opening of the Cicatrix Maledictum. Both the Khan and the Council know that Chaos, now more than ever, is trying to break into the Library.

 

There aren't many better guardians for such a place than one of the Emperor's own sons. Once again bound against his will he remains there, guarding the Library until such a time he can return to real space and rejoin his sons in the endless wars plaguing the Imperium of man.

 

Your turn!

Edited August 21 by Kaldoth

[Ulfast]

My headcanon is that it exist still a clone of Fulgrim that is loyal and true to the Imperium and one day, he will escape from Bile and help the Imperium against chaos. 

[Lathe Biosas]

I have a feeling that If Khan was in the Black Library, one of the Inquisitors that visited would have noticed him hanging out.

 

Inquisitor Czevak passed all the trials, and was there with the Eldar's permission and pretty much wandered around willy-nilly, while Inquisitor Draco and his team snuck in to the Black Library to see the Book of Fate... Neither mentioned a Khan-like figure... but maybe they had other things on their mind.

 

☆☆☆

 

My favorite bit of headcannon/crazed conspiracy I that appeared at 3 a.m. revolves around the mysterious alien spacecraft that elevated the Tau from primitive to extraordinary. 

 

All of the Tau's technology comes from one crashed starship (see the anime Macross for more information) they discovered by "chance" and apparently wasn't there the last time the Mechanicus rolled through. (Odd that the Mechanicus who is doing detailed scans would miss a giant spaceship sitting on a moon)

 

The "theory" says the Cabal (remember those fun guys from the Horus Heresy who were trying to fix the future to be the way they wanted it to be) planted the ship there (so the Tau could easily find it) in an attempt to manipulate future outcomes, since humanity refused to play along. A young race like the Tau could be more easily lead, they just needed a better technology base to influence the universe around them.

Edited August 21 by Lathe Biosas
John Grammaticus appeared from the Webway to tell me what to write.

[Kaldoth]

4 minutes ago, Lathe Biosas said:

I have a feeling that If Khan was in the Black Library, one of the Inquisitors that visited would have noticed him hanging out.

 

Inquisitor Czevak passed all the trials, and was there with the Eldar's permission and pretty much wandered around willy-nilly, while Inquisitor Draco and his team snuck in to the Black Library to see the Book of Fate... Neither mentioned a Khan-like figure... but maybe they had other things on their mind.

 

☆☆☆

 

My favorite bit of headcannon/crazed cinsoiracy ai read online at 3 a.m. revolve around the Tau.

 

All of their technology comes from one crashed starship (see the anime Macross for more information). 

 

The "theory" says the Cabal planted the ship there in an attempt to manipulate future outcomes, since humanity refused to play along. A young race like the Tau could be more easily lead, they just needed a better technology base to influence the universe around them.

 

Oof... I'm not well read on a lot of Inquisition lore. I did read the Draco books a long time ago. I thought they were considered retconned at this point? Czevak I've never heard of. Well, there goes my headcanon I guess  Or at least the Black Library part of it. He can keep pit fighting in my brain and waiting for a rescue 

 

Also, I'm a huge Macross fan! Happen to have a Glaug on my desk right now. I actually really like the idea of Cabal involvement. What was the motivation for them doing so, if you recall? They do tend to try and influence things on a grand scale. While the Tau are a powerhouse in their own right, against every other major player in the galaxy, IMO they're just a blip on the universal influence vs threat axis.

[Lathe Biosas]

Inquisitor Czevak, was featured in tons of codexes and blurbs spread throughout gw products for years. He only has 1 novel (IIRC) called Atlas Infernal by Rob Sanders.

 

 

Quote

From the Black Library website:

 

Inquisitor Bronislaw Czevak is a hunted man. Escaping from the Black Library of the eldar, Czevak steals the Atlas Infernal - a living map of the Webway. With this fabled artefact and his supreme intellect, Czevak foils the predations of the Harlequins sent to apprehend him and thwarts his enemies within the Inquisition who want to kill him. Czevak's deadliest foe, however, is Ahriman - arch-sorcerer of the Thousand Sons. He desires the knowledge within the Black Library, knowledge that can exalt him to godhood, and is willing to destroy the inquisitor to obtain it. A desperate chase that will bend the fabric of reality ensues, where Czevak's only hope of survival is to outwit the chosen of Tzeentch, Lord of Chaos and Architect of Fate. Failure is unconscionable, the very cost to the Imperium unimaginable.

 

But for fans of a certain age, he was our idea of what an Inquisitor should be like (Gregor Eisenhorn was never as cool).

 

☆☆☆

 

Going back to the Cabal... remember they were a bunch of aliens who were playing the long game, and while we had some idea of what they wanted and how it affected the near future, we knew they had contingency plans.

 

Maybe the Tau was one of them.

[ZeroWolf]

8 hours ago, Ulfast said:

My headcanon is that it exist still a clone of Fulgrim that is loyal and true to the Imperium and one day, he will escape from Bile and help the Imperium against chaos. 

Didn’t clonegrim get handed over to Trazyn though in one of the bile books

[SpecialIssue]

10 hours ago, Lathe Biosas said:

My favorite bit of headcannon/crazed conspiracy I that appeared at 3 a.m. revolves around the mysterious alien spacecraft that elevated the Tau from primitive to extraordinary. 

 

All of the Tau's technology comes from one crashed starship (see the anime Macross for more information) they discovered by "chance" and apparently wasn't there the last time the Mechanicus rolled through. (Odd that the Mechanicus who is doing detailed scans would miss a giant spaceship sitting on a moon)

 

The "theory" says the Cabal (remember those fun guys from the Horus Heresy who were trying to fix the future to be the way they wanted it to be) planted the ship there (so the Tau could easily find it) in an attempt to manipulate future outcomes, since humanity refused to play along. A young race like the Tau could be more easily lead, they just needed a better technology base to influence the universe around them.

 

One of my fav pieces of material is the book Xenology, which presents all its theories and autopsies on xenos from what is revealed to be an extremely unreliable narrator under the thrall / driven mad by Necrons / the C'tan so as to make it extremely deniable; and given it came out in 4th edition, is probably all superseded anyway. 

 

But yes, the remarks in that book on the pheromonal organs of the Ethereal caste and the completely incongruous nature with the rest of their physiology and evolutionary history, the pointing out of very similar (identical) organs in another insectoid alien species, and the sudden both appearance of the Ethereals, their organs, and technology in Tau society all point towards some sort of (Eldar is again intimated) outside influence.

Edited August 21 by SpecialIssue

[Cactus]

I wouldn't worry about inquisitors not mentioning the Khan - never assume that an inquisitor's report contains the whole truth.

 

My own head canon for the Tau dates back to when they were first released. I saw the hooves, and the Elementals' pheromone-based societal control and thought "new Tyranid infiltration species". Nothing yet has proved me wrong!

[Lathe Biosas]

2 hours ago, Cactus said:

I wouldn't worry about inquisitors not mentioning the Khan - never assume that an inquisitor's report contains the whole truth.

 

My own head canon for the Tau dates back to when they were first released. I saw the hooves, and the Elementals' pheromone-based societal control and thought "new Tyranid infiltration species". Nothing yet has proved me wrong!

 

What about Genestealer Cultists trying to murder them? Is it just a case of too much compartmentalization? Where one organization doesn't realise they are actually on the same side.

 

Now that I think of it, that makes a lot of sense in 40k land.

[Griefbringer]

On 8/21/2026 at 9:49 AM, Kaldoth said:

Was having a convo with a buddy of mine over the lore in 40k and just how many retcons, loose ends, and open avenues exist for the setting. We got into our favourite pieces of headcanon and it made me wonder what everyone else has going on in their brain. Is there an unresolved piece of the story you've got a theory about, or an ending to a loose thread you think would tie it up nicely?

 

My first thought on reading this was "Be careful about what you wish, for it might become true!" The setting has many open ends and mysteries (not to mention occasional contradictions), and a whole lot these are intentionally present, to tickle the reader's imagination. To quote the ancient overfiend Andy Chambers (WD 271, page 12):

 

"... Rick Priestley taught me an invaluable lesson early in my career. When you have a big, ever-expanding back story, it never pays to be too tidy with it. If you leave some plot threads unfinished and some enigmatic mentions unexplained you always have new elements to bring forward at later date and players have plenty of material to bring into their games if they like."

 

It would be a whole lot more boring setting if all of the mysterious open endings would be explained. Even the best explanation may be more boring than an interesting mystery. And perhaps the most interesting are those mysteries where people can easily come up with half dozen explanations, all of them somewhat plausible, and you cannot really pick which one you prefer.

 

 

On second thought, reading through the replies here, I liked the ones speculating about the origins of the mysterious Tau development (Mechanicum manipulation, Tyranid infiltration species, Eldar interference etc.).  As for the crashed spaceship hypothesis, while no doubt plausible, I would like to point out that the Kroot developed their technology from a crashed Ork rok, and it might be a tad boring if the Tau simply had similar origins of technological development. That said, the Kroot method of technological development involved eating Ork mekboyz and absorbing their technological knowledge (part of which is contained in the genetic structure of the Mekboyz themselves).

 

That said, it is a bit of a mystery why the various races of the galaxy, having developed at different places at different times, have sufficiently similar genetic built-up that Kroot are naturally able to absorb them all. Mind me, also the extra-galactic tyranids seem to be able to absorb that same genetic material, and genestealers are in principle able to infect a variety of species with their own genetic material.

Edited August 22 by Griefbringer
Fixed a typo

[ZeroWolf]

1 hour ago, Griefbringer said:

 

My first thought on reading this was "Be careful about what you wish, for it might become true!" The setting has many open ends and mysteries (not to mention occasional contradictions), and a whole lot these are intentionally present, to tickle the reader's imagination. To quote the ancient overfiend Andy Chambers (WD 271, page 12):

 

"... Rick Priestley taught me an invaluable lesoon early in my career. When you have a big, ever-expanding back story, it never pays to be too tidy with it. If you leave some plot threads unfinished and some enigmatic mentions unexplained you always have new elements to bring forward at later date and players have plenty of material to bring into their games if they like."

 

It would be a whole lot more boring setting if all of the mysterious open endings would be explained. Even the best explanation may be more boring than an interesting mystery. And perhaps the most interesting are those mysteries where people can easily come up with half dozen explanations, all of them somewhat plausible, and you cannot really pick which one you prefer.

 

 

On second thought, reading through the replies here, I liked the ones speculating about the origins of the mysterious Tau development (Mechanicum manipulation, Tyranid infiltration species, Eldar interference etc.).  As for the crashed spaceship hypothesis, while no doubt plausible, I would like to point out that the Kroot developed their technology from a crashed Ork rok, and it might be a tad boring if the Tau simply had similar origins of technological development. That said, the Kroot method of technological development involved eating Ork mekboyz and absorbing their technological knowledge (part of which is contained in the genetic structure of the Mekboyz themselves).

 

That said, it is a bit of a mystery why the various races of the galaxy, having developed at different places at different times, have sufficiently similar genetic built-up that Kroot are naturally able to absorb them all. Mind me, also the extra-galactic tyranids seem to be able to absorb that same genetic material, and genestealers are in principle able to infect a variety of species with their own genetic material.

If I remember correctly wasnt that Andy Chambers comment made with regards to developing the C'tan when Necrons came out in 3rd edition? I always took that advice to heart

[Griefbringer]

1 hour ago, ZeroWolf said:

If I remember correctly wasnt that Andy Chambers comment made with regards to developing the C'tan when Necrons came out in 3rd edition?

 

Sounds like you have an excellent memory! That comment is indeed from the Necron Designers' Notes article for the original Codex Necrons, and from the section of the article dealing specifically with C'tan.

 

Notice the plural form in Designers' - besides Andy Chambers, also Pete Haines, Phil Kelly, Graham McNeill and Andy Hoare have a bit to see about their contibutions to the book. Jes Goodwin also gets to have a few words about the revised look of the (now plastic) Necrons models, together with a few drawings from his sketch book. It is an excellent designers' notes article in that it really discusses the thinking and the work behind the book (rather than drumming about how strong the army is on the table top).

 

Maybe I should also cite Andy Hoare's brief contribution: "Finally ... I have some insight into the secrets of the Old Ones! For me, this was the best part of the project, pulling all the threads together and seeing 'big picture' of what's really going on in the Warhammer 40,000 universe. Of course, as soon as the details had been worked out they were hidden again, but most are there in the codex, and plenty more are still tucked away for the future. Conspiracy theorists will have a field day!"

 

I think that nicely complements what the Overfiend Andy said earlier about leaving mysteries for the reader to wonder about.

[ZeroWolf]

I remember pouring over the lore in that codex and associated WD articles, piecing together storythreads that I swore was there, like how much "help" the Deciever gave Abbadon just to make sure the Eldar couldn't use the Blackstone fortresses. It was a great time for storytelling. 

[N1SB]

On 8/21/2026 at 8:35 PM, Ulfast said:

My headcanon is that it exist still a clone of Fulgrim that is loyal and true to the Imperium and one day, he will escape from Bile and help the Imperium against chaos. 

 

On 8/22/2026 at 4:59 AM, ZeroWolf said:

Didn’t clonegrim get handed over to Trazyn though in one of the bile books

 

Brother Ulfast, you can make this.  Fulgrim in a Necron list, Necrons have Blackstone and anti-Warp tech.

 

Fulgrim could be represented by a Transcendant C'tan.  If there's 1 dude who can finagle his way with something like Necrons, it's actually him; he had grown up on Chemos, a corporate world, worked his way up to be their equivalent of the global CEO, he can maneuver between Necron High Lords to get an actual army to fight Chaos.

[Griefbringer]

10 hours ago, ZeroWolf said:

I remember pouring over the lore in that codex and associated WD articles, piecing together storythreads that I swore was there, like how much "help" the Deciever gave Abbadon just to make sure the Eldar couldn't use the Blackstone fortresses. It was a great time for storytelling. 

 

Back in the days of WH40K 3rd edition, the authors of the background material tended to put effort into representing a lot of it from an in-universe perspective, that was by default incomplete (and at times biased), rather than from the more conventional "eye of god" perspective of complete information. This naturally contributed to the sense of mystery. Soon after also Warhammer (The Game of Fantasy Battles) partially adopted similar approach with their 6th edition material. Admittedly, in both cases the new material in the rule/army books started to gradually drift towards the "eye of god" perspective.

 

__________________________________________________________________________________________________________________________________________________

 

On another note, I am wondering if some readers come to this thread looking for information on head cannons rather than headcanon (many people tending to mix up cannon and canon when writing). Especially since head cannons have been a thing in the 30K/40K setting at least since the days of the original Adeptus Titanicus, when Warlord titans could mount weapon heads with a variety of guns (or even an extra chainsaw blade). Bigger ork gargants could also have a cannon jutting out of their head - or alternatively from an additional turret built above the head (hat cannon?). Perhaps the subject of head cannons could deserve a discussion of its own somewhere else on these forums.

[Lathe Biosas]

I've given up on spelling anything correctly. The magical spell correction annoyance on my phone likes to change normal words into weirdness. It loves to change "for" into "fir" and "its" to "it's."

 

But I digress... when I see "head cannon," I wonder why their's a gun in the toilet.

[Griefbringer]

14 minutes ago, Lathe Biosas said:

II wonder why their's a gun in the toilet.

 

Looks like your automagical spell correction thingy also likes to change there is into their's.

 

I am trying my best to avoid any software which automatically tries to change my spelling - I have spent so much time learning how to spell things, that I am not convinced that a piece of software automatically knows better what I mean. Then again, as a non-native speaker, for whom English is primarily a written than spoken language, my writing errors tend to differ quite a lot from those of native speakers (who seem to particularly struggle with homophonic words).

[Griefbringer]

On 8/22/2026 at 6:42 AM, Lathe Biosas said:

What about Genestealer Cultists trying to murder them? Is it just a case of too much compartmentalization? Where one organization doesn't realise they are actually on the same side.

 

Do the genestealers and their broods (clans/cults) actually know that they are a tyranid pre-cursor / infiltrator species? After some thinking, my impression is that they are not generally aware of having any relationship with the tyranids (except maybe when a hive fleet arrives to the close proximity), and there is actually no need for them have such awareness or knowledge. Rather, it could be in theory detrimental, if the "prey species" could somehow extract the information out of them.

 

Genestealers are perfectly able to spread around the galaxy without any interaction with the hive mind. Their strong instincts to breed, spread, hide and gain control of societies (especially effective with humans) lets them to obtain good positions in planets with plenty of life on them. Once succesful enough, they just need to subconsciously trigger the warp beacon, signalling "dinner here" message to prospective hive fleets.

[Lathe Biosas]

In the books, the Patriarch is hyper aware of the endgame, and what's going to happen to them, because to a tyranid, there is no concept of souls or permadeath. You are just consumed, reformed, and sent back out.

 

Now, the rank and file Cultists, even the hybrids have zero idea that they are going to be ripped usunder, turned into a slurry and drunk through giant straws into awaiting bio ships in orbit of their world.

[Griefbringer]

1 hour ago, Lathe Biosas said:

ripped usunder, turned into a slurry and drunk through giant straws into awaiting bio ships in orbit of their world.

 

Particularly lucky ones might get the privilege of skipping the part with giant straws. The early tyranid codices (2nd / 4th edition) tell of the early encounters with hive fleet Kraken, one of which was on the feral planet Lamarno, which had come completely under the sway of genestealer infestation:

 

"When a Tyranid hive fleet arrived the fierce tribesmen had calmly boarded the bio-ships to be consumed by their new living gods."

 

Which suggests that several ships had actually bothered to land on the planet, rather than choosing the more convenient approach of staying in the orbit, bombarding planet with mycetic spores, and then lowering the drinking straws to recover everything.

[Gamiel]

One of the Shakespire plays that the Imperium have complete texts for is The Phantom of Menace.

[Gamiel]

Most Imperial transports are promethium / battery hybrids and can function without liquid fuel but then only at lower speed. You need the promethium for flying starts and to get at higher speeds, driving with the liquid fuel engine also recharges the batteries.

[Evil Eye]

>Space Marines mostly hover around 7.5 ft tall; there are some HUGE Astartes but these are the exception rather than the rule. Artistic renderings depicting them all as 8-9 ft giants are Imperial propaganda paintings intended to make them look even more impressive than they already are.

>To add to this, Primaris are on the whole slightly taller than Firstborn on average, but usually only by about a few inches or so, and as Astartes arms and armour is produced to requirements in the Chapter forges, it is entirely possible for a predominantly Primaris Chapter to be clad in pre-Cawl power armour and vice versa. If all wearing identical armour, you would struggle to tell 10 Firstborn from 10 Primaris in a group photo.

>As with all Imperial technology, Space Marine wargear, but especially vehicles, is incredibly varied in appearance and construction techniques to the point even the same basic pattern of Rhino (Mk. IIC for instance) built by two different Chapters will be quite different beyond simple colour and decoration. Locally available materials and resources, the whims of the Masters of the Forge and the particular requirements of the Chapter will result in a great deal of major and minor variations. For instance, some Chapters will be sure the ceramite armour of their vehicles is polished (relatively) smooth, whilst others may leave them with a rougher, more "raw" finish like you'd expect on a WW2 tank, and so it is entirely appropriate to give your Space Marine vehicle minis armour texture or not depending on what you prefer. (I love armour texture on tanks!)

>There are also a great deal of obscure, locally-restricted or semi-lost patterns of vehicles, weapons and armour used by the Astartes, and even seemingly defined things like power armour marks have a large amount of sub-patterns and varieties. Furthermore there are some very weird "spinoff" armour marks which are not counted amongst the main power armour family tree, as they were never standardized across the Imperium and many weren't even fully sanctioned by the Mechanicus (such as the forboding "Menhir" armour of the Death Crusaders with its distinct semi-fixed helmet dome).

>The Adepta Sororitas have a surprising amount of flexibility in religious doctrine and regalia; whilst theoretically intended to be relatively homogenized in this regard due to their role as defenders of the faith and the military arm of the Ecclesiarchy, the Imperium is such a huge and fragmented place with so much infighting and divergences of dogma even within the central Imperial Cult that in practice, the Sisters of Battle vary almost as much as the Space Marines in precise beliefs, aesthetics and indeed fighting style. This is also amplified in the case of Orders intended to bring the light of the Emperor to far-flung planets and more primitive worlds, where the exact details (but not core values) of their teachings are altered slightly to help assimilate cultures into the Emperor's flock.

>Ork kolour kultur, whilst mostly purely kultural (I greatly dislike the flanderization of "purple Orks are literal invisible") is a lot more complicated than just "red wunz go fasta". Notably, combinations of different colours have different meanings, some simple, some more esoteric. As just one simple example, red and purple camoflage on a Kommando would signify speedy, sneaky stabbin' skillz, being able to krump his enemies with great swiftness and without being noticed; in effect, an Orky ninja. _{Either that or he just really liked G2 Swindle.}

>Genestealers can predate upon/infect other species than humans, but usually require specific strains to do so. So Genestealers evolved to infect Orks can only infect Orks, etc.

[Bloodwraith]

My headcanon is Dorn is not "missing" The fight he dissappeared in was on an Alpha Legion vessel. The Alpha Legion and the Iron Warriors worked together and created a trap for Dorn. Perturabo and his forces with Alpha Legion reinforcements capture Dorn but he loses his hand in the fight. The Alpha Legion wanted him because they wanted revenge for him killing Alpharius and elicited the aide of the Iron Warriors and Perturabo. Perturabo was eager to aid becuase of his deep rooted hatred/jealousy for/of Dorn. The plan was to capture him and than the Alpha Legion would take him prisoner and use him to manipulate several future plans against the Imperium.

 

But once he was captured, the Iron Warriors turn on the Alpha Legion and betray and kill most of them. Perturabo takes Dorn prisoner and takes him to Medrengard to the Infinite citadel as a prisoner. There over the last 10000 years he has tortured and mocked Dorn trying to break him. But the only answers he ever got from dorn was Yes or No and he did not know of the Unbreakable Steel Will Dorn showed to Khorne. Meantime Dorn has used the time as Prisoner to listen, everytime Perturabo or one of his Warsmiths interrogated him, Dorn listened, they would inevitably let slip a portion of their plans out of arrogance for the situation Dorn was in. Dorn calculated his escape and in current lore as Perturabo launches his new offensive, Dorn finally after 10000 years, escapes and is making his way slowly to Terra!!!

Edited Tuesday at 10:52 AM by Bloodwraith

[LSM]

To cross-post from this thread on Fabius Bile's ties to the Cursed Founding:

 

The facilities on Incunabla pre-date the Cursed Founding, and are in fact an old lab of Fabius' from the Great Crusade/Heresy. (Their door marked with the Imperial Eagle - the symbol given to the Emperor's Children.) It's gene-tech was considered bad enough to be kept hidden by the Inquisition, but not so bad that it needed to be detroyed. Renegade and Traitor, but not Chaos. Cloning machines, research into the original creation of the Space Marines (and how they might be improved), etc.

 

Inquisitor Crescere (who kept their ressurectionist beliefs secret, it being heavily disfavoured pre-Sebastian Thor) managed to be in a position of power during the 21st Founding (overseeing the purity of the new Chapters' gene-seed) and decided to use Bile's five-millenia-old lab to try to engineer the Emperor a new body. He told the geneticists that they were merely secretly creating a new generation of superior Space Marines at the site - a smaller indiscretion, to hide the bigger one. And Crescere didn't tell them who their predecessor at Incunabla had been, until he was called to reclaim what was his.

 

The foundational work of the Cursed Founding was therefore the experiments of Fabius Bile, which is why those surviving Chapters bear the same Genetic Corruption as Bile's Enhanced Warriors.