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I know, I know... these types of threads have been made in multitudes, but I would like to discuss it further.

Primarch/Legion II and XI have been sealed/redacted, so not very many people know what exactly happened to them, save a few that were in the Great Crusade and are still alive, with even fewer are willing to talk about it.

Let's start with the possibilities:

1) Mutations/corruption/genetic instability was a pretty common thing when it came to some of the Space Marine Legions with examples of Magnus' and Fulgrim's Legions being on the brink of collapse due to various mutations and genetic screw-ups. If the Lost Primarchs/Legions had the same thing happen to them, why cover it up, but not do the same for Magnus and Fulgrim?

2) Chaos/Renegade Primarchs/Legions are also not unusual, as we have seen throughout the Horus Heresy series that 9 of the 18 remaining Legions turned on the Imperium. They were declared excommunicate traitoris, but knowledge of them still is out in the open. If the Lost Primarchs and their respective Legions were accused of heresy, why redact/remove information on them, when the Horus Heresy was undeniably worse?

So if mutations, heresy, corruption, what have you, doesn't fit the narrative, what possible reason could they have been "lost"? If they weren't executed by the likes of the Space Wolves, then something tragic must of happened to them, which should have been memorialized, not covered up.

Going back to the people who were in the Great Crusade and are still alive, why not ask them? Roboute Guilliman has sworn to never mention them, but Bjorn the Fell-Handed, Belisarius Cawl, the Fallen, Daemon Primarchs and their Legions would absolutely know a name, a color, a symbol that is related to these Lost Primarchs and their respective Legions. What do they have to lose? Nothing and even more so for the Daemon Primarchs and their Legions. They would love to spread the knowledge if they were heretics just like them.

Games Workshop's original intention was to make Legion II and XI your own Legion/Chapter, but as the lore has been expanded in great detail, there is no getting out of this.

Thoughts?

I have a huge writeup on this somewhere, but, short form that isn't supported by the canon as it stands?

Legion II was lead by Athena, the lone daughter of the Emperor. He Legion was formed out of Geneseed-tweaked men like the others, but she had a mindset based on transforming conquered societies, where she served as a teacher and healer instead of just a valiant warrior. A builder as it were.

She was one of the Legions directed "west" in the Pacific Sector, but that sector's small for a reason … the legions engaged in it where all pretty peaceful types (The Sons of Athena, the Salamanders, and the Word Bearers( who believed in making certain that the worlds they took over stayed loyal and understood the greater mission, but it was slooooow.

Eventually, the Emperor pushed for them to do more, so Athena turned to SCIENCE! and tapped her friend, and fellow healer, Apothecary Bile, from the Emperor's Children (She and Fulgrim got along great) … he helped switch up the gneseed to work in women, thus giving her twice as many warriors to work from. This lead to a boom in her expansion, so the Emperor came to pay a visit to find out why she was suddenly doing better. When he saw female Marines, he flipped … after all, if there were both men and women of the Astartes line, they could REPLACE humanity, which he was very much against.

Not only was her legion (Now renamed the Sons and Daughters of Athena) ordered destroyed, but all records were to be purged so that no one would ever know that it was even *possible* to do this.

A few fled and survived, going underground, and the Heresey happened soon after, so the Emperor was a tad busy and never able to finish the job before he got throned, so there's still a small cult out there, the Daughter of Athena, who fight on against the Imperium and keep up a low level guerilla war in the sector, where the Salamanders and the Rainbow Warriors just can't ever seem to quite catch them, dog gone it. Looks like they slipped away again. So CRAFTY those Athenites!

Warrior Woman of Schaeffer's Last Chancers is a member of the Cult of Athena, and other aspects of the story (Athena's own geneseed had nasty defects so was locked away until used in the Cursed Founding, then locked away again for obvious reasons, where the Sons of Medusa and Rainbow Wariors come from, the Starchild) are mixed in as well to try and solve a few unanswered questions.

It's from, man, twenty years ago or so? OLD work of mine, but still neat.

It all started with two questions: Why aren't there female marines and What did Legion #2 do that was WORSE than turning to Chaos?

So, that's my pitch. The full version's around 4000 words long I think. It's BIG and has way more detail in it.

Not even GW knows what happened to them. They were just 2 random entries to make background feel deeper and more thought out than it actually was.

And now GW can't really reveal if they are smart as whatever they come up with would be illogical and anti-climatic. They work better as what they are. Closed doors

One of the things that attracted me to 40k was the mystery of the lost primarchs. Have been working on head cannon for a while now and tried my best to find out what happens from all the hints.

It seems the II probably had a warp related accident or a Catastrophe from some unknown tech. Fulgrim mentioned something similar I think but can’t think where it came from.

The XI seems to have been killed in The Ragdan xenocides(maybe even the II as well) as more info is coming out on the wars and mentionS that entire legions were lost. This is the story I am working on for the IX, and using the Primarch tarot card for inspiration.

But still doesn’t answer why they covered it up? It is maddening haha. I know it is a long shot but I think one or both were dangerous to the Emperor, maybe some kind of ability, or psychic power that could potentially hurt/kill him.
Also have theorized that one of the primarchs altered either himself and/or his marines in a way that threatened the Emperor or the Great Crusade.

Edit; also believe one Primarch could have had a mutation that was contagious to other marines not of their gene-seed, which could have killed all the legions. That would be a great idea for a story..

I honestly think it was never meant to be revealed. It was a quirk of the system to add some intrigue. And it worked.

Anything GW releases will be a retcon and will damage the universe IMHO.

In my headcanon, one of them was the actual successor to the Emp and has been hidden away for *reasons* . The other, following the Ying/Yang style of the Primarchs was the literal embodiment of Chaos and has been hidden away for *reasons*.

I'm good with the odd unexplained bits of a story, but still think the big E did some Men in Black mind tinkering on a vast scale to cover it up and thats why hh era characters cant/wont explain it as all they know is 2/11 did something bad

Gav Thorpe was on MWG the other day and the question was asked. They're based on missing Roman legions that were expunged from records. There's nothing that's been decided and hidden, they're just one of the anomaly's of the setting that nobody will ever know (unless they decide to cash in on them I guess). I know Gav has to give his business answer, but I didn't detect a hint of excitement from him knowing something we don't.

At this point, nothing GW could come up with would live up to the expectation, unfortunately. After so many years of this being a big mystery, they'd have to come up with an explanation that's seriously mind-blowing, or people are just going to be disappointed...
I hope they won't try to milk that mystery for cheap suspense either. There are few things more frustrating than writers pretending to have some big reveal while they're just emptying a trawler full of red herrings... (It's been decades and I'm still peeved about The X-Files )

The missing legions are great fodder for headcanon and that's about all they should be.

 Psionara wrote:
I know, I know... these types of threads have been made in multitudes, but I would like to discuss it further.

Primarch/Legion II and XI have been sealed/redacted, so not very many people know what exactly happened to them, save a few that were in the Great Crusade and are still alive, with even fewer are willing to talk about it.

Let's start with the possibilities:

1) Mutations/corruption/genetic instability was a pretty common thing when it came to some of the Space Marine Legions with examples of Magnus' and Fulgrim's Legions being on the brink of collapse due to various mutations and genetic screw-ups. If the Lost Primarchs/Legions had the same thing happen to them, why cover it up, but not do the same for Magnus and Fulgrim?

2) Chaos/Renegade Primarchs/Legions are also not unusual, as we have seen throughout the Horus Heresy series that 9 of the 18 remaining Legions turned on the Imperium. They were declared excommunicate traitoris, but knowledge of them still is out in the open. If the Lost Primarchs and their respective Legions were accused of heresy, why redact/remove information on them, when the Horus Heresy was undeniably worse?

So if mutations, heresy, corruption, what have you, doesn't fit the narrative, what possible reason could they have been "lost"? If they weren't executed by the likes of the Space Wolves, then something tragic must of happened to them, which should have been memorialized, not covered up.

Going back to the people who were in the Great Crusade and are still alive, why not ask them? Roboute Guilliman has sworn to never mention them, but Bjorn the Fell-Handed, Belisarius Cawl, the Fallen, Daemon Primarchs and their Legions would absolutely know a name, a color, a symbol that is related to these Lost Primarchs and their respective Legions. What do they have to lose? Nothing and even more so for the Daemon Primarchs and their Legions. They would love to spread the knowledge if they were heretics just like them.

Games Workshop's original intention was to make Legion II and XI your own Legion/Chapter, but as the lore has been expanded in great detail, there is no getting out of this.

Thoughts?

A little fuel to the fire: https://www.reddit.com/r/40kLore/comments/b3dc0u/shortstory_excerptthe_chamber_at_the_end_of/

 Andersp90 wrote:
A little fuel to the fire: https://www.reddit.com/r/40kLore/comments/b3dc0u/shortstory_excerptthe_chamber_at_the_end_of/

There's also "The Last Council", which is mentioned further down that Reddit page.

 Psionara wrote:
Going back to the people who were in the Great Crusade and are still alive, why not ask them? Roboute Guilliman has sworn to never mention them, but Bjorn the Fell-Handed, Belisarius Cawl, the Fallen, Daemon Primarchs and their Legions would absolutely know a name, a color, a symbol that is related to these Lost Primarchs and their respective Legions. What do they have to lose? Nothing and even more so for the Daemon Primarchs and their Legions. They would love to spread the knowledge if they were heretics just like them.

Not even if they were heretics, just something that showed the Emperor to be a fraud, weak or imperfect would suffice.

 Psionara wrote:
Games Workshop's original intention was to make Legion II and XI your own Legion/Chapter, but as the lore has been expanded in great detail, there is no getting out of this.

Yea, I never got that one. The "master tome", aka the Rogue Trader book, is quite clear in that there are 1000 chapters, yet they need to keep 2 founding legions "spare" so you can make your own?
It always felt as if it was a case of "wizards did it", because someone asked, no one knew, and they wanted to give an answer to get people to stop asking.

"The galaxy is huge...

there are thousands of explanations for every question

all of them are wrong

believe whichever lie makes you feel the best"

Flavor I remember from back in Rogue Trader and 1st ed days

 Psionara wrote:

Games Workshop's original intention was to make Legion II and XI your own Legion/Chapter, but as the lore has been expanded in great detail, there is no getting out of this.

Sure there is, they simply don't produce any lore for them. There, problem solved - and from what I understand this is precisely the case with those two legions, the design team and Black Library have purposefully *not* written any explanation or lore behind what happened to those two legions and have only written "around" them in order to - as vaguely as possible - make it clear that they once existed and are now missing (which is why some of the statements made about them and some of the information that fans have linked to them, appear to be self-contradictory, because the writers aren't actually working with any real fluff, just putting out noise to call attention to their absence).

Yea, I never got that one. The "master tome", aka the Rogue Trader book, is quite clear in that there are 1000 chapters, yet they need to keep 2 founding legions "spare" so you can make your own?

Makes perfect sense, the 1000 chapters give you leeway to create your own chapter, BUT by default that chapter has to be a descendant of one of the original legions/primarchs which puts a degree of constraint on what it is you're able to write and how you justify it. By offering up 2 missing legions, you now have the opportunity and freedom to tie your chapters history to something else which you have created as well. Want your custom chapter to be weresharks, because their primarch was a mutated shark-man? There, now you can do it.

chaos0xomega wrote:
Want your custom chapter to be weresharks, because their primarch was a mutated shark-man? There, now you can do it.

But due to geneseed mutations, you can do that anyway, from any parent Legion.

I give you the Cursed 21st Founding as a prime example.

Whilst the results were caused by insufficiently skilled Mechanicus meddling, at least 1 Chapter came from known stable stock.

As a second example, I give you the Blood Ravens, a Chapter who do not know their genetic source.

So having "missing" Primarchs isnt entirely necessary.

Fictional wrote:

 Psionara wrote:
Games Workshop's original intention was to make Legion II and XI your own Legion/Chapter, but as the lore has been expanded in great detail, there is no getting out of this.

Yea, I never got that one. The "master tome", aka the Rogue Trader book, is quite clear in that there are 1000 chapters, yet they need to keep 2 founding legions "spare" so you can make your own?
It always felt as if it was a case of "wizards did it", because someone asked, no one knew, and they wanted to give an answer to get people to stop asking.

Wasn't it an Adeptus Titanicus / Space Marine / Epic thing (which was set in the HH) rather that Rogue Trader?

Fictional wrote:

chaos0xomega wrote:
Want your custom chapter to be weresharks, because their primarch was a mutated shark-man? There, now you can do it.

But due to geneseed mutations, you can do that anyway, from any parent Legion.

I give you the Cursed 21st Founding as a prime example.

Whilst the results were caused by insufficiently skilled Mechanicus meddling, at least 1 Chapter came from known stable stock.

As a second example, I give you the Blood Ravens, a Chapter who do not know their genetic source.

So having "missing" Primarchs isnt entirely necessary.

The piece you missed:

because their primarch was a mutated shark-man

Sometimes you want your spacemen to be just like your daddy spaceman and not a bunch of rejects with defective geneseed.

Fictional wrote:

 Psionara wrote:
Games Workshop's original intention was to make Legion II and XI your own Legion/Chapter, but as the lore has been expanded in great detail, there is no getting out of this.

Yea, I never got that one. The "master tome", aka the Rogue Trader book, is quite clear in that there are 1000 chapters, yet they need to keep 2 founding legions "spare" so you can make your own?
It always felt as if it was a case of "wizards did it", because someone asked, no one knew, and they wanted to give an answer to get people to stop asking.

It isn't actually true, which is why it doesn't make sense.

Founding legions (or, indeed legions at all) aren't in Rogue Trader. The Space Marineconcept is very different (drugged out criminals and maniacs, leashed by brainwashing and more drugs, of the various organs they're famous for, only the black carapace is referenced. The rest is just "their bodies are toughened by bio-chem, and their resolved is heightened by psycho-surgery"),

Organizationally, while a dozen are pictured, and named, they don't line up with founding chapters. RT notes that each chapter has its own history, rituals and traditions, and many have been in existence since the Age of the Imperium began, and that individual chapter commanders are free to organizse or augment their regular troops in any way they see fit. The readers are left with 988 chapters to fill out for themselves at the time of Rogue Trader.

Its interesting to note the Spacewolf fortress gets a four page write-up as an example of the 'typical space marine base,' and they're nothing like the Spacewolves of today. They're monks, with places for prayer and meetings, and a strict schedule for such things. The fortress is the domain of Imperial Commander Enoch, ruler of the planet Lucan and leader of the Space Wolves. (Masters of the chapter are a lesser title). Leman Russ is referenced- as the founder, but also the pilot of the spacecraft 'Medusa,' which hangs in the great hall. He also has a picture earlier in the book (heavily scarred, rebreather, cybernetic eye) with the following caption:
"Marine Commander;
Imperial Record AA/SW05/015 f19P&. Profile: Leman Russ, Born 2612016, M32., Guranta D Gurantan system.
Commissioned Adeputs Terra as special agent 0134041, M32. First rose to imperial notice during Lucan Crusade. Appointed Imperial
Commander Lucan 0333042, M32. Instrumental in founding Adeptus Astartes unti 4 'Spacewolves.' Suffered severe alviola damage during
acid storms on Susa. Transplanted with a model cybron-osmotic gill"

Not sure if its a typo for unit or if 'unti' means something else, but it may be a reference to Spacewolves as the 4th Adeptus Astartes unit (something the iron warriors might object to).

----
Anyway, the 12 chapters pictured in Rogue Trader are
Dark Angels (in their all black armor)
Flesh Eaters
Flesh Tearers
Spacewolves (one word)
Ultra Marines (two words- they get fleshed out in RT-era products like Warhammer Siege as not a full SM chapter until the Battle of Macragge, when they moved to that planet permanently, honored for defeating the tyranids). Also for a while they have a half-eldar psyker in their ranks
White Scars

Blood Angels
Blood Drinkers
Crimson Fists
Iron Hands
Rainbow Warriors
Silver Skulls

Not appearing in this book- Imperial Fists, Raven Guard. Also not in this book (wouldn't get fleshed out til realm of chaos: slaves to darkness)- any concept or even mention of the heresy and chaos marines, none of that is in RT, so no legions. The Emperor is in the Throne because of the strain of looking after humanity, nothing else.

That's back when Sanguinus was killed by a Bloodthirster at the gates of the Imperial Palace, rather than on the barge, and also when the battle between the Emperor and Horus was decided by Ollanius Pius, a plain ol' guardsman who stood up to Horus and, while killed SAAGELY, gave the Emperor the drive to finish it.

Ol' OP was a man's man if ever there was one but his place in the story's been purged.

Alas, alas.

They're monks, with places for prayer and meetings, and a strict schedule for such things.

Interesting - does this imply the possibility that Space Marines, as a collective whole, owe their modern interpretation to the Rogue Trader era Spacewolves?

tneva82 wrote:
Not even GW knows what happened to them. They were just 2 random entries to make background feel deeper and more thought out than it actually was.

And now GW can't really reveal if they are smart as whatever they come up with would be illogical and anti-climatic. They work better as what they are. Closed doors

Kinda close, but not quite there.

The two anonymous Chapters (for that was their title at the time. Love my old background, this isn’t meant to be a pointless flex) were anonymous to allow hobbyists room to create their own.

Now, as for the canonical reason? I can only put it down to the Primarchs themselves being heavily mutated in body. Like, heavily heavily mutated. The Emperor accepted Sanguinius despite his wings. He accepted (well, abused) Angron despite his serious headgear.

Other Primarchs had pronounced psychic abilities, even if they chose to repress them.

So whatever flaw or sin the lost Primarchs committed must be mind boggling. Like, seriously mind boggling.

It could even be as innocuous as being more psychically potent than The Emperor - in the same way as my dear old Daddykins is 5’8”, yet my brother and I are 6’4” and 6’2” respectively. Quirks of genetics and that.

 Mad Doc Grotsnik wrote:

tneva82 wrote:
Not even GW knows what happened to them. They were just 2 random entries to make background feel deeper and more thought out than it actually was.

And now GW can't really reveal if they are smart as whatever they come up with would be illogical and anti-climatic. They work better as what they are. Closed doors

Kinda close, but not quite there.

The two anonymous Chapters (for that was their title at the time. Love my old background, this isn’t meant to be a pointless flex) were anonymous to allow hobbyists room to create their own.

Now, as for the canonical reason? I can only put it down to the Primarchs themselves being heavily mutated in body. Like, heavily heavily mutated. The Emperor accepted Sanguinius despite his wings. He accepted (well, abused) Angron despite his serious headgear.

Other Primarchs had pronounced psychic abilities, even if they chose to repress them.

So whatever flaw or sin the lost Primarchs committed must be mind boggling. Like, seriously mind boggling.

It could even be as innocuous as being more psychically potent than The Emperor - in the same way as my dear old Daddykins is 5’8”, yet my brother and I are 6’4” and 6’2” respectively. Quirks of genetics and that.

It could be they went traitor or just plain failed. Or things could be boring and they could have just been killed before they were found.

Whatever happened to them, appears to have happened to their Legion.

Which raises the interesting idea that their Geneseed damned them. For whatever reason, it proved untenable. And so The Emperor just wiped the slate?

pm713 wrote:

 Mad Doc Grotsnik wrote:

tneva82 wrote:
Not even GW knows what happened to them. They were just 2 random entries to make background feel deeper and more thought out than it actually was.

And now GW can't really reveal if they are smart as whatever they come up with would be illogical and anti-climatic. They work better as what they are. Closed doors

Kinda close, but not quite there.

The two anonymous Chapters (for that was their title at the time. Love my old background, this isn’t meant to be a pointless flex) were anonymous to allow hobbyists room to create their own.

Now, as for the canonical reason? I can only put it down to the Primarchs themselves being heavily mutated in body. Like, heavily heavily mutated. The Emperor accepted Sanguinius despite his wings. He accepted (well, abused) Angron despite his serious headgear.

Other Primarchs had pronounced psychic abilities, even if they chose to repress them.

So whatever flaw or sin the lost Primarchs committed must be mind boggling. Like, seriously mind boggling.

It could even be as innocuous as being more psychically potent than The Emperor - in the same way as my dear old Daddykins is 5’8”, yet my brother and I are 6’4” and 6’2” respectively. Quirks of genetics and that.

It could be they went traitor or just plain failed. Or things could be boring and they could have just been killed before they were found.

I'm doubtful that either missing primarch turned traitor - if the Emperor knew his primarchs could/would rebel he'd have put some sort of strict oversight/control in place before the Heresy.

Unless we're going with the view of Emps as a 5 dimensional chess master that planned for the Heresy to happen.

chaos0xomega wrote:

They're monks, with places for prayer and meetings, and a strict schedule for such things.

Interesting - does this imply the possibility that Space Marines, as a collective whole, owe their modern interpretation to the Rogue Trader era Spacewolves?

Not really. The text is pretty clear that this is just a typical chapter- they could have grabbed anyone. They just had the picture page for Leman Russ (cybernetic gills and all) earlier in the book, so they had a convenient name and planet to throw them on.
The distinctions between chapters have another edition before they start surfacing.

pm713 wrote:It could be they went traitor or just plain failed. Or things could be boring and they could have just been killed before they were found.

The various hints dropped in the HH series make it clear that isn't the case. While Corax never met them, it seemed pretty clear some of the others had- its implied very heavily that the Space Wolves wiped out at least one of them on orders.

Plus the Crusade went on for over 200 years, and their elimination is dated to 43 years before the Istvaan Massacre. So they overlapped with a fair bit of the great crusade's history.

Way back in the late 90’s our group theorized that Sigmar was one of the most Primarchs. There were even little Easter eggs left in supplements to encourage the idea. This was before the HH series, when Primarchs where mysterious and mythic brings. This Theory was supposedly officially debunked by GW.
I stumbled upon this video a while back, this guy does a great job with the idea, bringing it in the modern 40k and AoS era.

https://youtu.be/ApmVcV0a2wI

That was a pretty popular/common theory. I believe Sigmar/Archaon (not sure if that was covered in the video, didnt watch) were theorized to both be primarchs due to the supposed interconnectivity of the two settings early on, but as time went on I think it became apparent this wasn't the case as GW became adamant that they were two separate settings that shared common themes (mainly the Warp/chaos gods).

Other than that the Valedictors and the Rainbow Warriors seem to be the only theory that has any legs (as they were named as being one of the original 20 in early sources before being retconned to being Ultramarines successors.

 ScarletRose wrote:

pm713 wrote:

 Mad Doc Grotsnik wrote:

tneva82 wrote:
Not even GW knows what happened to them. They were just 2 random entries to make background feel deeper and more thought out than it actually was.

And now GW can't really reveal if they are smart as whatever they come up with would be illogical and anti-climatic. They work better as what they are. Closed doors

Kinda close, but not quite there.

The two anonymous Chapters (for that was their title at the time. Love my old background, this isn’t meant to be a pointless flex) were anonymous to allow hobbyists room to create their own.

Now, as for the canonical reason? I can only put it down to the Primarchs themselves being heavily mutated in body. Like, heavily heavily mutated. The Emperor accepted Sanguinius despite his wings. He accepted (well, abused) Angron despite his serious headgear.

Other Primarchs had pronounced psychic abilities, even if they chose to repress them.

So whatever flaw or sin the lost Primarchs committed must be mind boggling. Like, seriously mind boggling.

It could even be as innocuous as being more psychically potent than The Emperor - in the same way as my dear old Daddykins is 5’8”, yet my brother and I are 6’4” and 6’2” respectively. Quirks of genetics and that.

It could be they went traitor or just plain failed. Or things could be boring and they could have just been killed before they were found.

I'm doubtful that either missing primarch turned traitor - if the Emperor knew his primarchs could/would rebel he'd have put some sort of strict oversight/control in place before the Heresy.

Unless we're going with the view of Emps as a 5 dimensional chess master that planned for the Heresy to happen.

Some people do have that view.

The fact they could rebel was obvious from square 1. They were people. Plus the Emperor is not that smart.

Personal headcanon whenever questions about the lost primarchs is asked, (Not to be taken seriously )

The two lost primarchs landed far from the reach of the Emperor and the Imperial forces, on backward worlds struggling against a foul xenos menace. One favoured brutality above all means, but was able to utilise it in a cunning method. The other was more cunning in nature, but could still fight in brutal ways. Their resistance against the foul xenos inspired both friend and foe in the ongoing struggles. Word reached the Emperor too late, and he began to feel the early foundations of their actions growing within the vast alien gestalt consciousness. With a heavy heart he knew that while they remained loyal to humanity, their ongoing presence and exploits would merely add fuel to their status so their termination was ordered as quickly as possible..

A drastic purge of all information regarding the two lost brothers was conducted, and the remaining primarchs were sworn to secrecy, never to utter their names again. Their respective legions were fed into the most extreme warzones, utilising their talents, but with recruitment stopped and casualties mounting, they faded into obscurity and were disbanded into other legions.

But their memory could not so easily be removed from the xenos. Over the centuries, facts became myths, myths became legends. Who they were or even that they were once human was forgotten in the mists of time. Only their methods of war and larger than life personalities survived. And unusually their curious way of speaking low gothic took hold in the masses, sounding like the utterings of soccer hooligans from ancient Terra. The only thing we know was that their names once began with the letters G and M, but even the xenos have corrupted the names over the centuries in their own fashion.

Nice I vaguely remember the idea that the survivors like the Loyalists from the Traitor Legions became Ultramarines which explained why they reached the numbers they had before Calth.

Originally, the lost legions were said to have fought in the Heresy, and probably on Horus's side. That led to the obvious conclusion that they were traitors who had been completely destroyed, and so their records were expunged to complete their punishment, effectively completely erasing them from existence. The same wasn't done for the other traitor Legions because they were all still running around... erasing them from Imperial records would have made fighting against them problematic.

With the HH series retconning the Lost Legions to being gone before the Heresy went down, that theory no longer works, but Sanguinius drops a fairly solid hint in one of the books that mutation was to blame for at least one of them, as he tries to conceal his own Legion's flaw specifically in order to avoid the same fate. So the obvious theory then becomes that these Legions were too flawed to be viable, and were erased from records to maintain the illusion that the Legions are perfect.