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The two missing Primarchs. We've never learned anything about them, but maybe there's some info in what we haven't seen.

A classic concept of the Primarchs is that each Primarch represents an aspect of the Emperor. Gulliman is the Emperor's drive to build an empire, Magnus is the Emperor's psychic might, Jaghatai Khan is the Emperor's conquering warrior spirit, etc. There are two sides that we see in many of the Primarchs, which are mirrored by the Emperor, that aren't exemplified by any particular Primarch.

The Emperor as a geneticist.
The Emperor as a monster.

These are two rather important aspects of the Emperor's story. He's created and modified humanity many times over his existence, most notably the creation of the Primarchs, Space Marines, Custodes, and Thunder Warriors. Yet no Primarch exists which exemplifies this aspect. Odd, no?
The other is the Emperor as a Monster. This is perhaps the most hidden aspect, but one that is also plainly visible if you look. The Emperor is not human, nor are any of his creations. They are all for the benefit of humanity, but they themselves are oft considered monsters.

There is yet another way to look at the Primarchs though, and this is one that I don't know if many have thought about. I've certainly never seen it written in an article (yet, but if you know of one that does, let me know as I'd love to read it!). Many, though not all, Primarchs are responsible for the existence, or are the epitomes, of a certain organization within the Space Marines. Magnus is responsible for Librarians, Lorgar for the Chaplains, Ferrus for the Tech Marines.

One very prominent organization without such a connection are the Apothecaries. And wouldn't you know it, but that has some insane cross-over with the concept of the Emperor as a geneticist. I think we can safely say that one of the "Forgotten" Primarchs was a geneticist, and committed an act of unforgivable genetic modifications. Likely creating humans that were no longer human, or making himself more xenos than man. Whatever it was, it likely had to do with genetic purity, just as the Apothecaries do.

As for the Primarch of Monsters, there are no Monsters in the ranks of the Space Marines...

Heck yeah there are.

Not even counting the monsters that were recently "fixed" by Cawls Primaris marines (the Blood Angels black rage, the Wolve's curse of the Wulfen), each Chapter has those recruits that do no survive the enhancement process to become a Space Marine. This most commonly occurs with the implantation of the Gene Seed. The result of these failures are monstrous mutants which the Chapter puts down immediately.

I think it's pretty safe to say that the other "Forgotten" Primarch was a Primarch of Monsters; a Primarch that wasn't really human, maybe totally feral and murderous. Likely, such a Primarch's Gene Seed was just totally unusable for the creation of Space Marines, turning almost all of them into monsters as well. It would be the Emperor's Failure, and it is no wonder then that the Primarch, any Space Marines created from that Geneseeed, and even the record of that Primarch's existence, would be expunged.

Neat theory, no?

I like this, but I think both lost legions had to be in active service with their primarchs for at least a short time.

The monster might have only had the full extent of his marines' mutant nature revealed after a while had passed, something that the Blood Angels and Space Wolves would be very careful to avoid.

The geneticist and his legion could hang out doing normal crusade stuff until things really hit the fan also.

Maybe since the Rangdan Genocides were connected at least vaguely to the timing of one legion's disappearance, the legion became unacceptably monstrous and modified in order to counteract the Rangdan bio-weapons/mind control/etc?

whatever polishes your pistol my friend. my theory is that they ran out of ideas but didn't like 18 as it wasnt a round enough number. call me cynical..I don't know.

Lore wise the 2 lost legions are hinted at and subtly mentioned in the HH novels.(cant remember the sources) and its suggested that other legions were used to destroy them... possibly the SW. prospero burns maybe?

 queen_annes_revenge wrote:
my theory is that they ran out of ideas but didn't like 18 as it wasnt a round enough number. call me cynical..I don't know.

I think the original idea was to let people create their own Legions.

yeah I've heard that floated too, but I dont think having 2 missing legions is really necessary to allow home brew chapters etc.

In Clonelord it's hinted that one of the missing Primarchs lead an expedition into the Ymga Monolith, itself a Necron warp-dampener. Recognizing the size and scale of the Necron threat to the nascent Imperium seems like something that might be suppressed in order to avoid a panic.

Rick Priestly answered the original intent for the missing legions in an interview with Baron Bifford. He said that they were initially just added to include a sense of mystery. No make your own legion, no reference to lost Roman legions. The intent was to eventually fill in who the missing legions were and why they were removed, but Priestly left GW before it became a thing.

There's actually a ton of anecdotal evidence that suggests that the 2nd and 11th failed in some fashion. One, if not both, were likely destroyed by the Space Wolves.

1. In Deliverance Lost, Corax wants to know how he is the 19th Primarch, if there are only 18. The Emperor gets all mopey and tells hm that it's a conversation for another day. This tells us that they're gone before Corax is found.
2. In A Thousand Sons, Magnus starts to comment on a time that nine or more Primarchs last gathered. Mortarion shuts him down, saying , "And the Emperor forbade us to speak of it again. Do you disobey that command?".
3. In Fear to Tread, Sanguinius justifies to Horus why he's never told the Emperor about the Red Thirst... "You know the reason! [...] I will not be responsible for the erasure of the Blood Angels from Imperial history. I will not have a third empty plinth beneath the roof of the Hegemon as my Legion's only memorial!" We know from previous books that the two empty plinths are the 2nd and the 11th Primarchs. This strongly implies that the 2nd and 11th were 'erased' because of some inherent flaw.
4. The First Heretic is a little sketchy, because the narrative is being related by an unreliable Chaos source, but Magnus seems to confirm that the Emperor was responsible for removing the 2nd and 11th.
5. In False Gods, Horus slams his fist down on the gestation chamber of the 11th Primarch, damaging it shortly before it was swept out of the Imperial palace and scattered through the warp. It's very possible this damage caused the nascent Primarch to have been tainted by Chaos and never actually given command of a Legion.
6. In the First Heretic, it's noted that the Ultramarines Legion swelled in size at around the time that the 2nd and 11th Legions vanished. It's implied that the Marines were just given blue paint jobs. If point 5 above is correct, the 11th would never have known their Primarch, so rolling to the Ultramarines would have been a non issue.
7. In Prospero Burns, Leman Russ is asked if he's worried about the coming battle with the Thousand Sons. He notes that this won't be the first time they've been sent to censure a Legion. The thought is that the Wolves took out the 2nd Legion, assuming the 11th was rolled into the 13th.
8. In the Outcast Dead, one character notes that "the Wolves will be loosed again" when talking about the sanction of the Thousand Sons. Again, evidence that they've taken down a Legion before.
9. In Vulkan Lives, Vulkan says, "I am [troubled] Artellus. Very much so. None of us wants another sanction, another empty pillar in the great investiary, another brother's name excised from all record. It is shame enough to bear the grief for two. I have no wish to add to it, but what choice do I have?" Again, veiled confirmation that the missing two Primarchs were sanctioned by the Emperor.
10. In the Unremembered Empire, Guilliman and El'Johnson bicker about how many seats should be at a table made for Primarchs. Guilliman has 21... 20 Primarchs plus 1 seat for the Emperor. He notes that it's still a matter of honor to leave a seat open, even if you know it will never be filled. Hence, Guilliman and El'Johnson are aware of the 2nd and 11th and know why they're gone.
11. In Dark Imperium, Guilliman says, "I was one of twenty. Two failed. Half the rest turned on my father. The Emperor is not infallible, nor am I." Again, he knows what happened. He comments that the two missing Primarchs failed.

There's more, but that's all I'm willing to look for right now.

In short, the Primarchs who were found by the Emperor early on know exactly what happened to the 2nd and the 11th. The ones who were found later, Corax and Alpharius/Omegon maybe, don't know. In other words, the erasure from history was so effective that not even a Primarch was really aware of what happened. Whatever happened, the Emperor authorized it and the Wolves most likely enacted the censure/punishment/mercy kill or whatever the specific situation called for.

My personal assumption is that the 2nd led his Legion and somehow screwed the pooch. The Emperor got pissed and called in the Wolves. I think the 11th was damaged by Chaos as an infant and never led his Legion. The 11th Legionnaires would have been rolled in with the Ultramarines. If true, at least a portion of current Ultramarine genestock may not come from RG.

That is very strong evidence in favour of a "Monster Primarch", but is neither in favour or not in favour of a Geneticist Primarch. Thanks Kriswall! I haven't read any of the Horus Heresy books; just know what I know from a long history with the game and reading Lexicanum and whatnot.

When you say "monster primarch" do you mean one who'd be physically inhuman, through mutations or something? Because if you're just looking for the cruel, violent and terrifying side of the "monster", Curze already fills that niche pretty well.

Personally, I like to think that one of the primarchs was so revolted by all the massacres the Emperor's Great Crusade required that he decided to make a stand at some point. Maybe he declared himself the protector of some civilization marked for extermination and was wiped out with them.

Tiennos wrote:


Personally, I like to think that one of the primarchs was so revolted by all the massacres the Emperor's Great Crusade required that he decided to make a stand at some point. Maybe he declared himself the protector of some civilization marked for extermination and was wiped out with them.

Yes.. yes.. YES!!!!!! I like it!!

I have a belief that the 2nd realised what a terrible being the big E really is (he is only interested in webway/knowledge and couldint give 2 gaks about humanity). So he realised there was no happy ending for humanity from big E's viewpoint so decides to try and save some real humans civilisation. I have an idea that he took a bunch of people and decided to gtfo of the milky way and do an exodus. And one day they will return to liberate humanity from IOMs shackles and the taint of chaos!!!

The last council pretty must states outright that at least one of them failed in some way and was purged for it. No other details are really given other strong hints that most of the primarchs were given some level of mental block that made it hard to think about them.

What's always made me curious was even before the HH, several legions were pretty insubordinate and they didn't get purged.

The Nightlords and World Eaters were borderline uncontrollable, Magnus flaunted the emperors authority on multiple occasions before it came to calling the wolves (which wasn't meant to be a fight till Horus mucked that up) and Lorgar was pretty much off the reservation as well. None of these guys were in line to get purged, so what ever 2 and 11 did must have been really foul.

beast_gts wrote:

I think the original idea was to let people create their own Legions.

It wasn't. This is an idea that people have frequently suggested over the years, and in recent years one or two of the BL authors have suggested it as a reason, but it was never even mentioned as a thing back in the day. It was purely there to add mystery to the setting.

Having them there to let people make their own doesn't even make any sense. Having them deleted from all records doesn't work if they're still active in the Imperium... someone has to be overseeing them, unless they're traitors (as was suggested in the original fluff) in which case it still wouldn't make sense to have deleted them and not the other traitor legions. It also would have been uneccessary, since the Chapter setup already allowed people all the freedom they needed to create their own backgrounds.




Originally, it was stated that nothing much was known about them other than that they probably fought on Horus's side in the Heresy. That led to the most likely theory being that they were traitor legions who had been completely wiped out - thus, erasing them from all records completes their destruction.

With the HH novels changing that to them having been gone since long before the Heresy even started, it all became much more ambiguous, and I suspect that it will stay that way at least until well after the Heresy story is done with and the BL needs some other guaranteed money-spinner.


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HoundsofDemos wrote:
The last council pretty must states outright that at least one of them failed in some way and was purged for it. No other details are really given other strong hints that most of the primarchs were given some level of mental block that made it hard to think about them.

What's always made me curious was even before the HH, several legions were pretty insubordinate and they didn't get purged.

The Nightlords and World Eaters were borderline uncontrollable, Magnus flaunted the emperors authority on multiple occasions before it came to calling the wolves (which wasn't meant to be a fight till Horus mucked that up) and Lorgar was pretty much off the reservation as well. None of these guys were in line to get purged, so what ever 2 and 11 did must have been really foul.

Sanguinius' comments in [whichever book that was] strongly suggest that the issue for at least one of them was genetic, rather than behavioural.

HoundsofDemos wrote:
The last council pretty must states outright that at least one of them failed in some way and was purged for it. No other details are really given other strong hints that most of the primarchs were given some level of mental block that made it hard to think about them.

What's always made me curious was even before the HH, several legions were pretty insubordinate and they didn't get purged.

The Nightlords and World Eaters were borderline uncontrollable, Magnus flaunted the emperors authority on multiple occasions before it came to calling the wolves (which wasn't meant to be a fight till Horus mucked that up) and Lorgar was pretty much off the reservation as well. None of these guys were in line to get purged, so what ever 2 and 11 did must have been really foul.

I like the idea of outright rebellion. The Emperor's arrogant enough to let World Eaters and such have a pass because they still do what he wants but there's no way to spin rebellion as beneficial and I like the idea someone twigged the Emperor was nuts.

Not even counting the monsters that were recently "fixed" by Cawls Primaris marines (the Blood Angels black rage, the Wolve's curse of the Wulfen)

Are Primaris flawless then? No defects as above?
Thats a bit boring imho.

 Kriswall wrote:
I think the 11th was damaged by Chaos as an infant and never led his Legion. The 11th Legionnaires would have been rolled in with the Ultramarines. If true, at least a portion of current Ultramarine genestock may not come from RG.

So, those who trace their genestock back to the 11th (even if they don't actually know it themselves) may become tainted Primaris in future... *evil laugh*

 Argive wrote:


I have a belief that the 2nd realised what a terrible being the big E really is (he is only interested in webway/knowledge and couldint give 2 gaks about humanity). So he realised there was no happy ending for humanity from big E's viewpoint so decides to try and save some real humans civilisation. I have an idea that he took a bunch of people and decided to gtfo of the milky way and do an exodus. And one day they will return to liberate humanity from IOMs shackles and the taint of chaos!!!

Perhaps his name was Ksandr Kereneski. Or something like that.

I always fancied that Constantin Valdor may have been one of the missing 2.

Tiennos wrote:
When you say "monster primarch" do you mean one who'd be physically inhuman, through mutations or something? Because if you're just looking for the cruel, violent and terrifying side of the "monster", Curze already fills that niche pretty well.

Personally, I like to think that one of the primarchs was so revolted by all the massacres the Emperor's Great Crusade required that he decided to make a stand at some point. Maybe he declared himself the protector of some civilization marked for extermination and was wiped out with them.

Yeah, physically inhumane, even more a mutant than Magnus. Or maybe more were-wolf like in that it's a Primarch that undergoes a change during certain times, so perhaps appears human normally and then monstrous at other times. As for the Monster Primarch's temperment, that I do not know. It's tempting to say that perhaps it was a raw bestial savagery, but then again, perhaps not.

 Ratius wrote:

Not even counting the monsters that were recently "fixed" by Cawls Primaris marines (the Blood Angels black rage, the Wolve's curse of the Wulfen)

Are Primaris flawless then? No defects as above?
Thats a bit boring imho.

If you read the story, Cawl is extremely evasive throughout the whole conversation. The implication is that no, he didn't actually fix all of the flaws. Space Wolves will probably grow fangs as they age. Blood Angels will probably got a little crazy from time to time. Etc, etc.

Cool.

 Kriswall wrote:

 Ratius wrote:

Not even counting the monsters that were recently "fixed" by Cawls Primaris marines (the Blood Angels black rage, the Wolve's curse of the Wulfen)

Are Primaris flawless then? No defects as above?
Thats a bit boring imho.

If you read the story, Cawl is extremely evasive throughout the whole conversation. The implication is that no, he didn't actually fix all of the flaws. Space Wolves will probably grow fangs as they age. Blood Angels will probably got a little crazy from time to time. Etc, etc.

I can't remember if it was the wolves or blood angels, but I remember reading that one of them had hoped he had fixed their issues only to find out later that even with all his advancements there isn't a way to escape it.

HoundsofDemos wrote:

 Kriswall wrote:

 Ratius wrote:

Not even counting the monsters that were recently "fixed" by Cawls Primaris marines (the Blood Angels black rage, the Wolve's curse of the Wulfen)

Are Primaris flawless then? No defects as above?
Thats a bit boring imho.

If you read the story, Cawl is extremely evasive throughout the whole conversation. The implication is that no, he didn't actually fix all of the flaws. Space Wolves will probably grow fangs as they age. Blood Angels will probably got a little crazy from time to time. Etc, etc.

I can't remember if it was the wolves or blood angels, but I remember reading that one of them had hoped he had fixed their issues only to find out later that even with all his advancements there isn't a way to escape it.

Yeah. I don't remember the source, but I remember the same thing. I want to say that older Primaris Space Wolves (some are potentially 200+ years old at this point) are starting to get "long fangs". I'm also pretty sure the Primaris Blood Angels are subject to the Red Thirst (which seems to be physiological) and the Black Rage (which seems to be more of a psychic malady).

7. In Prospero Burns, Leman Russ is asked if he's worried about the coming battle with the Thousand Sons. He notes that this won't be the first time they've been sent to censure a Legion. The thought is that the Wolves took out the 2nd Legion, assuming the 11th was rolled into the 13th.

That's not really evidence, it could just be a reference to the Night of the Wolf where Russ and his Legion fought Angron and the World Eaters.

 insaniak wrote:
It wasn't. This is an idea that people have frequently suggested over the years, and in recent years one or two of the BL authors have suggested it as a reason, but it was never even mentioned as a thing back in the day. It was purely there to add mystery to the setting.

Having them there to let people make their own doesn't even make any sense. Having them deleted from all records doesn't work if they're still active in the Imperium... someone has to be overseeing them, unless they're traitors (as was suggested in the original fluff) in which case it still wouldn't make sense to have deleted them and not the other traitor legions. It also would have been uneccessary, since the Chapter setup already allowed people all the freedom they needed to create their own backgrounds.

Fun fact - they weren't actually unknown in the first edition. Alas, Rainbow Warriors didn't sound all that well, sooo...

 Kriswall wrote:
One, if not both, were likely destroyed by the Space Wolves.

Actually zero, because later fluff completely contradicts SW fighting other SM at that point.

6. In the First Heretic, it's noted that the Ultramarines Legion swelled in size at around the time that the 2nd and 11th Legions vanished. It's implied that the Marines were just given blue paint jobs. If point 5 above is correct, the 11th would never have known their Primarch, so rolling to the Ultramarines would have been a non issue.

No, that was just Word Bearer b-hurt Ultramarines were much more competently organized and recruited, trying to invent some conspiracy theory excusing WB shortcomings. We see UM organization in FW books and there are no 'foreign' elements.

7. In Prospero Burns, Leman Russ is asked if he's worried about the coming battle with the Thousand Sons. He notes that this won't be the first time they've been sent to censure a Legion. The thought is that the Wolves took out the 2nd Legion, assuming the 11th was rolled into the 13th.

No, that was furry meaning that failed attempt to rein Angron in and having his butt handed to him on silver platter. Funnily enough, both times he wasn't actually called, but decided to commit treason all on his own.

8. In the Outcast Dead, one character notes that "the Wolves will be loosed again" when talking about the sanction of the Thousand Sons. Again, evidence that they've taken down a Legion before.

That again refers to Angron/furry clash because that was explicitly stated to be first SM/SM confrontation, and that was after the two disappeared.

In short, the Primarchs who were found by the Emperor early on know exactly what happened to the 2nd and the 11th. The ones who were found later, Corax and Alpharius/Omegon maybe, don't know.

And yet, later fluff says they all met so that scene makes no sense. Most logical explanation is that Corax didn't saw two of his brothers after being found due to them being simply busy and censure coming much later down the line.

 Irbis wrote:

Fun fact - they weren't actually unknown in the first edition. Alas, Rainbow Warriors didn't sound all that well, sooo... .

They also weren't Legions in first edition, just Chapters... amongst many other differences before the current Space Marine origin story was fully established. Although I'm not sure there is enough information to say that they weren't 'unknown' at that point, since as far as I'm aware Rogue Trader never actually lists the first founding Chapters at all, just says that there were 20 of them.

And there has never been any suggestion that the Rainbow Warriors were a first founding Chapter, so I'm not sure why you brought them up.


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 Irbis wrote:
Funnily enough, both times he wasn't actually called, but decided to commit treason all on his own.

The Wolves were very clearly ordered to Prospero. The Emperor sent him, and sent along a bunch of Sisters of Silence and Custodes to help out, and then Horus amended Russ's orders, telling him to go in hard. Russ did exactly what he was ordered to do.

The horus heresy Black Books make mention of something during the Great Crusade called the Rangdan Xenocides.

It's rarely ever talked about much in any one place, but the sheer amount of references in the backgrounds of different legions and auxilia armies means it must have been one heck of a clusterfeth; there's a reference to it somewhere in every single black book.

The two critical references are this:
"Whole Expeditionary Fleets went to their deaths without a single survivor, worlds were laid waste, dozens of Titan Legions were obliterated and by the end, entire Space Marine Legions (exactly which Legions were affected was purposely redacted from Imperial records) had been lost to the Imperium."

and this:


The Rangdan Xenocides are probably facing down the original Slaugth empire given mention of 'Slaugth Murder-minds". Given how bloody terrifying they are in the Dark Heresy and Rogue Trader rpgs, and how good they were at manipulating and infiltrating societies, I can see them (and/or their original creators) being the big bads of the affair.

 insaniak wrote:
And there has never been any suggestion that the Rainbow Warriors were a first founding Chapter, so I'm not sure why you brought them up.

He might be thinking of the original 12 chapters to get official chapter icons in RT-era 40K publications and packaging (e.g. the RTB001 box, but also on the Space Crusade game board). Those were the Blood Angels, Blood Drinkers, Crimson Fists (Space Crusade replaced them with the Imperial Fists), Dark Angels, Flesh Eaters, Flesh Tearers, Whitescars, Ultramarines, Space Wolves, Silver Skulls, Iron Hands, and... Rainbow Warriors.

No, that was just Word Bearer b-hurt Ultramarines were much more competently organized and recruited, trying to invent some conspiracy theory excusing WB shortcomings. We see UM organization in FW books and there are no 'foreign' elements.

And the author outright said his intent was just word bearers pushing unfounded conspiracy theories rather then giving the Ultramarines their due. People need to stop taking every quote in the novels as the unfiltered truth, sometimes people are misinformed, and sometimes they even lie to the point of making elaborate conspiracy theories to discredit those they don't like.

 Duskweaver wrote:

 insaniak wrote:
And there has never been any suggestion that the Rainbow Warriors were a first founding Chapter, so I'm not sure why you brought them up.

He might be thinking of the original 12 chapters to get official chapter icons in RT-era 40K publications and packaging (e.g. the RTB001 box, but also on the Space Crusade game board). Those were the Blood Angels, Blood Drinkers, Crimson Fists (Space Crusade replaced them with the Imperial Fists), Dark Angels, Flesh Eaters, Flesh Tearers, Whitescars, Ultramarines, Space Wolves, Silver Skulls, Iron Hands, and... Rainbow Warriors.

To be fair, there was also a fan theory for some time that the Rainbow Warriors were one of the two missing Legions due to not being seen since Rogue Trader, combined with the old art piece of a Battle Sister 'purging' a Rainbow Warrior with fire.

That was never backed up by anything official, and then around 5th edition or so GW started showing Rainbow Warriors in amongst Chapter colour scheme collections again, which sort of blew that idea out of the water.

We know a reasonable amount about the missing Primarchs, enough to lay down some facts with some certainty.

We know that neither mystery Primarch betrayed or directly turned on the Emperor. We know this because of Guilliman's explicit separation in 'Dark Imperium'. ''"I was one of twenty. Two failed. Half the rest turned on my father. The Emperor is not infallible, nor am I." This is further supported by the general disbelief shown amongst the Legions at Horus' first betrayal, and a scene in 'Unremembered Empire' where Guilliman marks their passing by leaving chairs for the two missing Primarchs at a table. "Yet their absence must be marked. Places must be left for them. That is simply honor." He wouldn't have done this if they were traitors. Vulkan's statement in 'Vulkan Lives' is also not the sort of one you would apply to a traitor. "It is shame enough to bear the grief for two."

We know that at least one or possibly both Legions were lost in the Rangdan Xenocides. From the artwork linked above (which explicitly named them as taking part), and book seven of the Horus Heresy (which says entire legions were lost). Fun fact:- the Death Guard were also almost wiped out (from one of their character's fluff), and the Dark Angels lost 50,000 marines.

It is extremely heavily implied (if not outright stated by inference) the Rangdan had some means of biologically corrupting humans. We know this from the description of the Xenocides (Horus Heresy, Book VII). "What remained was for the Rangdan taint to be purged in a subsequent decade-long series of bio-pogroms that left entire human inhabited sectors lifeless to ensure what was hoped to be a final victory. It was then given to the Space Wolves of the VIth and the Dark Angels of the Ist - the latter who had suffered themselves so very dreadfully against the horror - to conduct these purges, these two Legions entrusted above all others to do what had to be done....In comparison to these new 'paragons', for the Space Wolves now came the whisper of 'executioner' rather than warrior, and the image of destroyer that had always been theirs in part now came to replace that of savage but noble conquerors in the minds of many in the Imperium." Given the many references scattered throughout the Heresy to the Space Wolves fighting other marines at some point, it would be a reasonable assumption that this took place during the Rangdan Xenocides, and that one (or two) of the missing Legions/Primarchs were thus corrupted by the Rangdan somehow.

This theory is strengthened by Sanguinius's fear of his own legion being purged for biological taint/deviance if the Black Thirst is mentioned to the Emperor (in Fear to Tread). ""I will not be responsible for the erasure of the Blood Angels from Imperial history. I will not have a third empty plinth beneath the roof of the Hegemon as my Legion's only memorial!" The theory is also further supported by Cawl's statement in 'Dark Imperium'. "The warriors were not at fault. The science is not at fault. Their Primarchs were." The implication being that (i) whatever happened to the missing Legion(s), it was as a result of a deliberate decision by the Primarch(s) involved and (ii) Whatever biological taint/issue resulted with the missing legion(s), it wasn't initially present but resulted from some later event.



In other words, with regards to one of them at least (and perhaps both), it's pretty clear what happened. Their Primarch made a decision, it resulted in his Legion being biologically tainted in some fashion, and the whole Legion had to be exterminated by the Wolves as a result. Not pleasant, and not pretty.