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 Harriticus wrote:
The existence of non-loyalist Primarchs is hidden from the Imperium at large. i.e. the traitor Primarch statues were taken down around the Imperial palaces

In real life, statues of traitors are taken down too without covering up the fact that they were traitors. There's like, a ton of other reasons why you'd want to remove traitor statues from your palace that have nothing to do with "keeping it a secret"

 morganfreeman wrote:

 winterwind85 wrote:
Chaos is not hidden from normal citizens... In the infantry mens uplifting Prime there are alot of Passages about chaos

The soldiery is different from ye average citizen of the Imperium. Despite how massive the armies of the Imperium are, the civilian population is much larger.

The church doubtlessly speaks of evil, corrupting heretics, and the demons luring the righteous from the path of the Emporer's Light.. But just in abstracts and watered down. Mention of the warp, Chaos itself, Nurgle, Khorne, Slaanesh, ect is not going to be even remotely present - because it would make some curious.

Exactly so. The Ecclesiarchy generically denounces Heretics and Daemon-Worshippers. What it doesn't do is mention specifics like the names of the fell powers or the fact that daemon-worshippers sometimes have their prayers answered.

The Sanguinalia is an Imperium-wide festival (mentioned in the 2nd edition rulebooks) commemorating Sanguinius' sacrifice - which implies that a reasonable proportion of citizens on duly Emperor-fearing and not too barbaric worlds know he exists, and (broadly) how he died, which means they know Horus existed and that he was a traitor.

Mind you, Horus is a convenient 'satan-equivalent' - incomprehensible to mere mortals, not all-to-clearly known to the average man himself and ten thousand years safely dead. You can villify him and his armies without, for example, mentioning that the astartes chapter supposed to be watching over your sector turned renegade a century ago and are lurking somewhere in the neighbourhood....

The knowledge of the invasion of Armageddon is suppressed by the Grey Knights not because a motley force of renegades and loony-worshippers turned up and attacked a world but specifically the arrival of daemons and (especially!) Angron.

To the best of my knowledge, the Primarchs are worshipped sort of like the Gods of Ancient Egypt were - that is, as Gods - while the Emperor is worshipped more like Amun-Ra was worshipped - that is, God of Gods. Horus is considered the Devil of Devils- even mentioning his name is bad luck, talking about him will bring about the Emperor's wrath (unless you're a part of the Eclessiarch). The average Imperial citizen is probably informed that Horus brought about the demise of most, if not all, of the Primarchs and the Primarchs are now Gods or Archangels themselves that will watch over you, under the guidance of the Emperor himself. Space Marines are distant Angel-Protectors that most believe in and most believe to be the extension of the Emperor's divine will.

Knowledge of Chaos is completely suppressed, even amongst the Imperial Guard. After Armageddon, the Inquisition mind-wiped the IG forces that fought there, almost leading to an all-out war with the Space Wolves that was only averted by Bjorn the Fell-Handed's good counsel to the Great Wolf.

 dusara217 wrote:
To the best of my knowledge, the Primarchs are worshipped sort of like the Gods of Ancient Egypt were - that is, as Gods - while the Emperor is worshipped more like Amun-Ra was worshipped - that is, God of Gods. Horus is considered the Devil of Devils- even mentioning his name is bad luck, talking about him will bring about the Emperor's wrath (unless you're a part of the Eclessiarch). The average Imperial citizen is probably informed that Horus brought about the demise of most, if not all, of the Primarchs and the Primarchs are now Gods or Archangels themselves that will watch over you, under the guidance of the Emperor himself. Space Marines are distant Angel-Protectors that most believe in and most believe to be the extension of the Emperor's divine will.

Knowledge of Chaos is completely suppressed, even amongst the Imperial Guard. After Armageddon, the Inquisition mind-wiped the IG forces that fought there, almost leading to an all-out war with the Space Wolves that was only averted by Bjorn the Fell-Handed's good counsel to the Great Wolf.

Lexicanum says the primarchs are de-facto "living saints", but doesn't give a source for it.

What is canonically known is that Roboute-in-stasis is visited by a ridiculous amount of pilgrims every year. It doesn't say if they see him as a god or not regarding that though.

It's been 10,000 years since the Horus Heresy. That is, roughly speaking, twice the length of recorded human history to the modern date in 2014.

The Ecclesiarchy could tell people anything about the Heresy, the Great Crusade, the Primarchs... and they wouldn't be necessarily wrong. The fact is, so few records survived that era, and so much time has passed, that the events of that time are mythical. Much like stories from the Bible, or the old tales like Beowulf, or the Saga of Roland, or the Tales of King Arthur, we have little to no concrete evidence that anything in those books actually happened the way the story says it happened (and rather a lot of evidence to suggest it didn't).

So it is with the Ecclesiarchy. They have a belief in the divinity of the God-Emperor they want to support, and will go to great lengths to ensure that their view is pretty much the only view (as broadly-defined as it may be) that the Imperium holds. Non-Imperial human worlds that are found receive missionaries from the Ecclesiarchy, who will reshape the local belief into something palatable to the Ecclesiarchy.

Worlds that resist this assimilation receive a War of Faith, and are purged.

Also, there is nothing that defines what the *exact* dogma of the Ecclesiarchy is, because there is no *exact* dogma. Generally speaking, the Ecclesiarchy believes that the Emperor was once a Man, and ascended to Divinity via the Golden Throne. The Primarchs are His sons, half of whom betrayed Him (the missing 2 are utterly unknown to just about everyone in M40), but were defeated. The remaining Primarchs are venerated as saints throughout the Imperium, as noted in sources for the Feast of Sanguinor, the pilgrimages taken to Ultramar to witness Guilleman, etc.

Specifics of how the Emperor is worshipped varies from planet to planet, as local religions were adopted into the Ecclesiarchy and re-shaped to fit their belief system... which is how Catholicism (upon which the Ecclesiarchy is based) worked during the Age of Exploration (and pretty much every other time, too).

Maximus Bitch wrote:
Oh man, some of these replies appear to have been spat out without much consideration.

Anyway, I believe

Not a good way to start a reply. 'I believe' means nothing, you need evidence.

Loving the fact you observe other people's posts as ill-conceived when yours suffers from the same flaws. At least theirs had some actual evidence in it, as opposed to 'I believe'.

It's a given fact that most of the population of the Imperium know little to nothing about Chaos, there's a reason Armageddon was repopulated with new colonists, and that was because they had a run-in with Angron. Any knowledge of the Chaos Primarchs is hidden or expunged, only to be revealed to the highest levels of Inquisitors, the Grey Knights and possibly a few other Marines who are too valuable to mind-wipe.

 BrotherOfBone wrote:

Maximus Bitch wrote:
Oh man, some of these replies appear to have been spat out without much consideration.

Anyway, I believe

Not a good way to start a reply. 'I believe' means nothing, you need evidence.

Loving the fact you observe other people's posts as ill-conceived when yours suffers from the same flaws. At least theirs had some actual evidence in it, as opposed to 'I believe'.

It's a given fact that most of the population of the Imperium know little to nothing about Chaos, there's a reason Armageddon was repopulated with new colonists, and that was because they had a run-in with Angron. Any knowledge of the Chaos Primarchs is hidden or expunged, only to be revealed to the highest levels of Inquisitors, the Grey Knights and possibly a few other Marines who are too valuable to mind-wipe.

I'm not sure about the "actual evidence", some of the replies were too brief or vague. Also, I was bemoaning the brevity of their replies. I provided more detail in my post, which at least demonstrates my process, and I started it with "Anyway", I was trying to provide an alternative viewpoint.

Yes, little to nothing, as I've mentioned before, but the Imperium still needs a bogeyman to keep its population complicit. A Daemon Primarch however, is too much of a bogeyman. The realization that the enemy has a Primarch, not just an "ordinary" one, but a daemon one, while you have none, is too demoralizing for most.

Unfortunately, Angron is depicted as a pale shadow of his 30K past, so as to let the good guys win a minor battle in the vast galaxy. As impotent as Angron is, a Daemon Primarch is still demoralizing as the Primarchs are portrayed as demigod-like saints to the Imperials.

Space Marines are too valuable and unnecessary to mind wipe. As for the Grey Knights, their secrecy is not against any loyal Imperial, since, unlike the Ordo Hereticus, they don't go around hunting their fellow human beings. It is to protect knowledge of their demoralizing J.D., which is like telling Imperials to expect the occasional daemonic incursion. And also to prevent their whereabouts from falling into the hands of traitors, who can proceed to sabotage them.

White Dwarf 151 provides some detail, but it was written in 1992. Still, fluff is fluid.




Automatically Appended Next Post:

 dusara217 wrote:
To the best of my knowledge, the Primarchs are worshipped sort of like the Gods of Ancient Egypt were - that is, as Gods - while the Emperor is worshipped more like Amun-Ra was worshipped - that is, God of Gods. Horus is considered the Devil of Devils- even mentioning his name is bad luck, talking about him will bring about the Emperor's wrath (unless you're a part of the Eclessiarch). The average Imperial citizen is probably informed that Horus brought about the demise of most, if not all, of the Primarchs and the Primarchs are now Gods or Archangels themselves that will watch over you, under the guidance of the Emperor himself. Space Marines are distant Angel-Protectors that most believe in and most believe to be the extension of the Emperor's divine will.

Knowledge of Chaos is completely suppressed, even amongst the Imperial Guard. After Armageddon, the Inquisition mind-wiped the IG forces that fought there, almost leading to an all-out war with the Space Wolves that was only averted by Bjorn the Fell-Handed's good counsel to the Great Wolf.

The Emperor is THE God, since he ascended after being placed on his golden toilet. The Primarchs are more like saints. Although some religious zealots today complain that saints are equivalent to polytheism.

And if Horus and Sanguinius were both favored sons, some might wonder how they ended up as angel and devil, such divergent paths. The answer is Chaos, presented in a non-destabilising manner. They might say that it is bad because it gives you an extra dick, man boobs, causes betrayal etc. Just like the devil whispered in Cain's ear and made him kill his brother Abel after God's rejection of his harvest. Though this is more applicable to Lorgar than to Horus.


Automatically Appended Next Post:

 Psienesis wrote:
It's been 10,000 years since the Horus Heresy. That is, roughly speaking, twice the length of recorded human history to the modern date in 2014.

The Ecclesiarchy could tell people anything about the Heresy, the Great Crusade, the Primarchs... and they wouldn't be necessarily wrong. The fact is, so few records survived that era, and so much time has passed, that the events of that time are mythical. Much like stories from the Bible, or the old tales like Beowulf, or the Saga of Roland, or the Tales of King Arthur, we have little to no concrete evidence that anything in those books actually happened the way the story says it happened (and rather a lot of evidence to suggest it didn't).

So it is with the Ecclesiarchy. They have a belief in the divinity of the God-Emperor they want to support, and will go to great lengths to ensure that their view is pretty much the only view (as broadly-defined as it may be) that the Imperium holds. Non-Imperial human worlds that are found receive missionaries from the Ecclesiarchy, who will reshape the local belief into something palatable to the Ecclesiarchy.

Worlds that resist this assimilation receive a War of Faith, and are purged.

Also, there is nothing that defines what the *exact* dogma of the Ecclesiarchy is, because there is no *exact* dogma. Generally speaking, the Ecclesiarchy believes that the Emperor was once a Man, and ascended to Divinity via the Golden Throne. The Primarchs are His sons, half of whom betrayed Him (the missing 2 are utterly unknown to just about everyone in M40), but were defeated. The remaining Primarchs are venerated as saints throughout the Imperium, as noted in sources for the Feast of Sanguinor, the pilgrimages taken to Ultramar to witness Guilleman, etc.

Specifics of how the Emperor is worshipped varies from planet to planet, as local religions were adopted into the Ecclesiarchy and re-shaped to fit their belief system... which is how Catholicism (upon which the Ecclesiarchy is based) worked during the Age of Exploration (and pretty much every other time, too).

I'd be more careful about grouping the Bible with Roland, Arthur and Beowulf.

The Horus Heresy is ancient, which is good for the Ecclesiarchy and Custodes as they can propagate a good name for their Emperor and savior. The Imperial administratum also likes a good propaganda tale to keep the population unified and in check. The Guard can use it as a good recruiting tool and a restorer of morale. Not too different from many societies on Terra right now.


Automatically Appended Next Post:

 Harriticus wrote:
The existence of non-loyalist Primarchs is hidden from the Imperium at large. i.e. the traitor Primarch statues were taken down around the Imperial palaces

But isn't the whole festival of Sanguinala centered around Sanguinius making a chink in Horus' armor and then dying against him, and then having the Emperor blast Horus through that hole (sounds wrong)

No, the Sanguinalia is about Sanguinius killing a bunch of monsters before being killed by the most powerful one, but giving the Emperor a chance to vanquish the beast.

Interestingly enough, many 'saints' actually are pagan gods, repurposed and rebranded for the sake of easier conversion to Christianity, for example Saint Brigit.

 Slaanesh-Devotee wrote:
I could accept that the might hear stories of one of his sons who turned against him in a fruitless rebellion, ala satan. That doesn't necessitate knowing about chaos, but I'm sure there's a heavy emphasis on how stupid it was and how easily and ruthlessly Horus was crushed.

This would be my basic guess. Horus was his favourite son, he got greedy and arrogant and tried to kill his father but Sanguinus (a good boy) gave his life to protect the Emperor, the Emperor slew his evil son and in an awesome display of power became a God, destroying the evil rebels (like the Ark scene in Raiders). Now respect your parents, be honest and humble, and eat your greens.

I imagine that's the story on the Anglican planet anyway.

Also yeah 10,000 years and a million worlds! There's probably a few trillion people who don't know much about Chaos in spite of the priests!

Just imagine you're someone who lives in the underhive - probably don't get many priests down there, the only Imperial authority you see are ultra-violent cops popping round every six months and your life is gak but without authoritarian control. No doubt there's plenty of gangers who think its cool to call the Imperial Cult bollocks. Right up until they get drafted and battle an endless horde of daemons.

There probably isn't a specific way that citizens view Primarch's. The Ecclessiarchy has to modify their approach for each culture they come across. Then the legends get even more distorted because of the lens they view through.

I believe it has been stated that the wider imperium don't know that the traitor primarchs were exactly that. They are aware of the existence of the arch heretic Horus but he and his commanders are largely mythic bogeymen. In Soul Hunter some of the characters don't even believe that there are such things as traitor astartes at first. Threats to the imperium are drastically downplayed and people are ignorant of chaos and xenos threats. This is why chaos and genestealer cults are such a threat to imperial worlds, by the time someone who knows enough to recognise the signs finds out about it it is often too late.

Maximus Bitch wrote:

Space Marines are too valuable and unnecessary to mind wipe.

It's because of their value they ARE mind wiped, instead of getting a bullet in the head.

a bit of old fluff:
"The only general exception to this policy of secrecy-by-extermination are Adeptus Astartes units. Execution of a Marine is seen as wasteful. Marine units are mindscrubbed rather than killed - their memories destroyed rather than their bodies. Mindscrubbing removes any and all memories of the Ordo's true purpose, but requires its victims to be completely retrained. Mindscrubbed Marines cannot even feed themselves, let alone fight for the Imperium." ~ Realms of Chaos: Slaves to Darkness page 247


and a bit of current fluff:
"The Red Hunters Chapter is tied to the Inquisition by ancient bonds of honour and duty. Fighting in Inquisitorial strike forces and providing honour guards for the most senior of Inquisitor-lords, the Red Hunters have confronted numerous horrors no mortals can face and be allowed to live should they survive, lest they taint others. As such, the Chapter’s line brethren are routinely mind-scoured to preserve their souls, a fact that makes them even more valuable weapons in the hands of the secretive and often necessarily brutal Inquisition. Through repeated mind scouring and deep-core psycho-indoctrination, Red Hunters brethren are instilled with countless subconscious battle doctrines which are only activated by the act of making war." ~ Forge World's 7th edition Chapter Tactics PDF

Mind scrubbing astartes must pose problems from time to time though, what if that astartes is Logan Grimnar or Commander Dante? You can't just mind wipe a chapter master. Not to mention how pissy a marine chapter would get if they lent the inquisition a battle company and got back a hundred drooling power armoured babies.

 Ugly Green Trog wrote:
Mind scrubbing astartes must pose problems from time to time though, what if that astartes is Logan Grimnar or Commander Dante? You can't just mind wipe a chapter master. Not to mention how pissy a marine chapter would get if they lent the inquisition a battle company and got back a hundred drooling power armoured babies.

Aren't Chapter Masters immune to the rule? IIRC they are part of the secret elite who know about Chaos and its true extent and nature. Also, there's always the Blood Drinkers whose every single fething member knows about the existence of Kairos Fateweaver. Given that there are also Chapters like the Primogenitors and the direct Legion descendants whose recorded history starts with, "Well, the Emperor founded us, then we found our Primarch, then we fought in the Great Crusade, then all feth happened and Horus went nuts and we fought in the Horus Heresy and such and such and such great heroes of the Chapter died fighting Chaos daemons," and continues with the equivalent of, "oh, and by the way, the DAEMON Primarch Magnus/Perturabo/Fulgrim/Lorgar hates our guts and we hate his guts, and we've been fighting him and his CHAOS SPACE MARINE LEGION for ages and ages again and again and again."

In addition to that, some Space Marines, especially Librarians and Chaplains, would by nature have to know about Chaos, especially given that's what they're there for. It would be utter stupidity to mindwipe a Chaplain - whose actual function is to prevent the Chapter falling to Chaos - to remove his knowledge of Chaos. Only the rank-and-file Marines would have been mindwiped, and the Chapter would probably simply chug them back into the original brainwashing machine they came out of as Neophytes and repeat the training process. I think it's already clear from fluff that the mindwiping of Space Marines exposed to Chaos follows special rules. Plus in the novels practically all Space Marines remember fighting Chaos elements at some points in their careers anyway, so they're not always mindwiped. Mostly they'd be mindwiped when major Chaos incursions happen or when the Grey Knights are involved, or when it's necessary to restabilize the sector after a major Chaos incursion and there's a need to rewrite history to suit the Inquisition's purposes.

Take Gabriel Angelos, who was a mere Battle Brother when he ratted out his HOMEWORLD to the Inquisition over its fall to Chaos. Planet got Exterminatus, but he wasn't mindwiped, and neither were his homies. Likely this was because all he saw were a bunch of crazed Chaos cultists dancing on the streets accompanied by a few summoned minor daemons rather than the more in-your-face horrific Chaos portal spewing out hordes of Greater Daemons of Chaos. It's a matter of degree, not of kind. There are levels of Chaos knowledge from the common ("Hey, Andy's been behaving weirdly lately, what with the piercings and the naked dancing and stuff") to the arcane ("Wait, what? How did he do that? That's impossible. Is that magic?") to the Eldritch ("Holy &%%$#^%$^# AAAAAAAA EMPEROR SAVE US!!!!"). Depending on who you were, someone might BLAM! you for knowledge of a particular level of Chaos activity. Battle Brothers might retain up to arcane level knowledge. After all, they see it all the time, even from their own Librarians (except Black Templars). Chapter Masters are probably required to know the Eldritch stuff, given some of them (*hint, hint, Great Wolf*) might even have to mano-a-mano Daemon Primarchs. Heck, in the HH novels even Primarchs had fragmentary knowledge of Chaos (John Grammaticus considered Alpharius' knowledge as dangerously childish).

As for popular knowledge of the Primarchs, fluff, especially in the novels, show that the general population are quite aware of the Primarchs' existence, that they were sons (as in REAL sons) of the Emperor, and that Sanguinius fell defending his Father from something. It's that "something" that they're fuzzy about. Also, given that it's hinted the Blood Angels and their successors' Chaplains occasionally debate the theology of Sanguinius' sacrifice (whether it was motivated by fatalism or courage) with Ecclesiarchy theologists, at least some priests are aware of Sanguinius' precognitive powers and retain some knowledge of how events unfolded in Horus' Battle Barge. Of course, this would be on the level of theological debates of whether Esther was being pious or deceptive when she argued for the safety of the Jews (Ahasuerus/Xerxes I wasn't monogamous and didn't have a Jewish wife, so this is a debate about the actions of a fictional character). However, the fallen and Daemon Primarchs are never mentioned by any non-military/Inquisitorial/high-ranking person in any of the fluff. It is then safe to say that the Imperial population in general are unaware of the existence of Traitor Primarchs. However, the Horus Heresy is a well-known event, simply in that it happened, so at least people would know about Horus.

Based on that, we could assume the following:
1. The Horus Heresy is actually common knowledge. Regular people do know the term, and they celebrate the Sanguinalia, so at least they have some idea of how it ended (except the part of what happened to the Emperor afterwards). You don't actually have to mention Chaos to make a good story of the Horus Heresy:

"The Emperor once had ten sons, of whom he favored Horus, the brightest, and Sanguinius, the mightiest - well, and maybe Guilliman, the bureaucratest - but lo, Horus, jealous of his Father's greatness rebelled, and he brought his Legions to bear against his brothers and his Father and on the holy soil of Terra they fought, and fell Sanguinius to his brother's betrayal, and yet before he passed truly, a chink made he upon his brother's armor. And when their Father, hallowed be His name, came upon his fallen son, great anger washed upon Him and smote He the great betrayer. And yet His heart was heavy, and His hands failed to deliver that killing blow, for truly had He loved His once-brightest son. And so Horus did the unspeakable deed and struck down his own loving, hallowed Father, and so close had he to fulfilling his dark destiny, when out of the ranks a single, lone soldier, a mortal no match for the mighty betrayer, stood his ground to defend his Emperor. And perished did he in but an instant, as the betrayer had lost all humanity and simply extinguished the brave one's existence. Yet this sacrifice it was that made the Emperor realize how far had His son fallen to treachery and evil, and He called upon his might and, through the chink in armor wrought by Sanguinius, smote for once and forever His once-beloved son. And so was the battle won and the Emperor returned in all His glory upon Holy Terra and seated Himself, to this day, upon the Golden Throne. Somewhere in all this Guilliman was doing something, and it was important. The end."

2. Nobody except those high up in the hierarchy and those Space Marines who have long-running grudges with the Traitor Legions actually know they exist, and consequently practically no commoner would even know of the existence of the Traitor Primarchs. No part of the official narrative actually requires the existence of the Traitor Legions and Primarchs to explain them. There's a reason it's called the HORUS Heresy, not the Horus and Lorgar and Angron and Perturabo and Alpharius and Fulgrim and Curze and Mortarion and Magnus Heresy. Blame it all on one guy.

3. All the other fallen Loyalist Primarchs' deaths can be chalked up to battlefield deaths without even mentioning Chaos. This is also partly helped by the fact that some are missing and in one case a dead one is considered missing by his sons. We 40K players might feel this is weird because we know the true extent of a Primarch's abilities (and that they're very hard to kill) but there would be no contradiction in the mind of the average Imperial citizen. Heroes die and saints are enshrined. That's normal.
- Guilliman fell in battle due to treachery. He WAS actually poisoned, so they don't even have to hide that fact. His body is preserved and on display, so no need to even make up stories about what happened to him.
- Dorn also fell in battle and his mummified fists are on display.
- Khan, Corax, and Russ went away and will return one day. If you don't believe this, the Scars, Ravens, and Wolves would like to have a chat with you.
- Vulkan is missing to the Salamanders. Well, WE know he's basically dead for real (and possibly being kept in Trazyn's vault), but don't tell them that. The Salamanders and the Promethean Cult think he's returning after all his relics are found. Due to the Salamanders being a depleted Legion after HH and their minor presence in the common Imperial citizenry's knowledge base, probably only Prometheans and a few even care that there was a Primarch known as Vulkan anyway.
- The Lion is on vacation. If you disagree, the Unforgiven's tea corner is always open for consultation.
- Ferrus Manus is dead, but his Iron Hands will turn you into a servitor if you tried to tell them that. Due to their stubborn insistence, most likely the mythology around him is fethed up beyond belief by the 41st Millennium anyway.

In conclusion , it's entirely possible for commoners throughout the Imperium to know about, discuss, and even pray for intervention from the Primarchs without Chaos or the Traitor Primarchs ever entering into their knowledge even once.

hurimirshugu wrote:

 Ugly Green Trog wrote:
Mind scrubbing astartes must pose problems from time to time though, what if that astartes is Logan Grimnar or Commander Dante? You can't just mind wipe a chapter master. Not to mention how pissy a marine chapter would get if they lent the inquisition a battle company and got back a hundred drooling power armoured babies.

Spoiler:

Aren't Chapter Masters immune to the rule? IIRC they are part of the secret elite who know about Chaos and its true extent and nature. Also, there's always the Blood Drinkers whose every single fething member knows about the existence of Kairos Fateweaver. Given that there are also Chapters like the Primogenitors and the direct Legion descendants whose recorded history starts with, "Well, the Emperor founded us, then we found our Primarch, then we fought in the Great Crusade, then all feth happened and Horus went nuts and we fought in the Horus Heresy and such and such and such great heroes of the Chapter died fighting Chaos daemons," and continues with the equivalent of, "oh, and by the way, the DAEMON Primarch Magnus/Perturabo/Fulgrim/Lorgar hates our guts and we hate his guts, and we've been fighting him and his CHAOS SPACE MARINE LEGION for ages and ages again and again and again."

In addition to that, some Space Marines, especially Librarians and Chaplains, would by nature have to know about Chaos, especially given that's what they're there for. It would be utter stupidity to mindwipe a Chaplain - whose actual function is to prevent the Chapter falling to Chaos - to remove his knowledge of Chaos. Only the rank-and-file Marines would have been mindwiped, and the Chapter would probably simply chug them back into the original brainwashing machine they came out of as Neophytes and repeat the training process. I think it's already clear from fluff that the mindwiping of Space Marines exposed to Chaos follows special rules. Plus in the novels practically all Space Marines remember fighting Chaos elements at some points in their careers anyway, so they're not always mindwiped. Mostly they'd be mindwiped when major Chaos incursions happen or when the Grey Knights are involved, or when it's necessary to restabilize the sector after a major Chaos incursion and there's a need to rewrite history to suit the Inquisition's purposes.

Take Gabriel Angelos, who was a mere Battle Brother when he ratted out his HOMEWORLD to the Inquisition over its fall to Chaos. Planet got Exterminatus, but he wasn't mindwiped, and neither were his homies. Likely this was because all he saw were a bunch of crazed Chaos cultists dancing on the streets accompanied by a few summoned minor daemons rather than the more in-your-face horrific Chaos portal spewing out hordes of Greater Daemons of Chaos. It's a matter of degree, not of kind. There are levels of Chaos knowledge from the common ("Hey, Andy's been behaving weirdly lately, what with the piercings and the naked dancing and stuff") to the arcane ("Wait, what? How did he do that? That's impossible. Is that magic?") to the Eldritch ("Holy &%%$#^%$^# AAAAAAAA EMPEROR SAVE US!!!!"). Depending on who you were, someone might BLAM! you for knowledge of a particular level of Chaos activity. Battle Brothers might retain up to arcane level knowledge. After all, they see it all the time, even from their own Librarians (except Black Templars). Chapter Masters are probably required to know the Eldritch stuff, given some of them (*hint, hint, Great Wolf*) might even have to mano-a-mano Daemon Primarchs. Heck, in the HH novels even Primarchs had fragmentary knowledge of Chaos (John Grammaticus considered Alpharius' knowledge as dangerously childish).

As for popular knowledge of the Primarchs, fluff, especially in the novels, show that the general population are quite aware of the Primarchs' existence, that they were sons (as in REAL sons) of the Emperor, and that Sanguinius fell defending his Father from something. It's that "something" that they're fuzzy about. Also, given that it's hinted the Blood Angels and their successors' Chaplains occasionally debate the theology of Sanguinius' sacrifice (whether it was motivated by fatalism or courage) with Ecclesiarchy theologists, at least some priests are aware of Sanguinius' precognitive powers and retain some knowledge of how events unfolded in Horus' Battle Barge. Of course, this would be on the level of theological debates of whether Esther was being pious or deceptive when she argued for the safety of the Jews (Ahasuerus/Xerxes I wasn't monogamous and didn't have a Jewish wife, so this is a debate about the actions of a fictional character). However, the fallen and Daemon Primarchs are never mentioned by any non-military/Inquisitorial/high-ranking person in any of the fluff. It is then safe to say that the Imperial population in general are unaware of the existence of Traitor Primarchs. However, the Horus Heresy is a well-known event, simply in that it happened, so at least people would know about Horus.

Based on that, we could assume the following:
1. The Horus Heresy is actually common knowledge. Regular people do know the term, and they celebrate the Sanguinalia, so at least they have some idea of how it ended (except the part of what happened to the Emperor afterwards). You don't actually have to mention Chaos to make a good story of the Horus Heresy:

"The Emperor once had ten sons, of whom he favored Horus, the brightest, and Sanguinius, the mightiest - well, and maybe Guilliman, the bureaucratest - but lo, Horus, jealous of his Father's greatness rebelled, and he brought his Legions to bear against his brothers and his Father and on the holy soil of Terra they fought, and fell Sanguinius to his brother's betrayal, and yet before he passed truly, a chink made he upon his brother's armor. And when their Father, hallowed be His name, came upon his fallen son, great anger washed upon Him and smote He the great betrayer. And yet His heart was heavy, and His hands failed to deliver that killing blow, for truly had He loved His once-brightest son. And so Horus did the unspeakable deed and struck down his own loving, hallowed Father, and so close had he to fulfilling his dark destiny, when out of the ranks a single, lone soldier, a mortal no match for the mighty betrayer, stood his ground to defend his Emperor. And perished did he in but an instant, as the betrayer had lost all humanity and simply extinguished the brave one's existence. Yet this sacrifice it was that made the Emperor realize how far had His son fallen to treachery and evil, and He called upon his might and, through the chink in armor wrought by Sanguinius, smote for once and forever His once-beloved son. And so was the battle won and the Emperor returned in all His glory upon Holy Terra and seated Himself, to this day, upon the Golden Throne. Somewhere in all this Guilliman was doing something, and it was important. The end."

2. Nobody except those high up in the hierarchy and those Space Marines who have long-running grudges with the Traitor Legions actually know they exist, and consequently practically no commoner would even know of the existence of the Traitor Primarchs. No part of the official narrative actually requires the existence of the Traitor Legions and Primarchs to explain them. There's a reason it's called the HORUS Heresy, not the Horus and Lorgar and Angron and Perturabo and Alpharius and Fulgrim and Curze and Mortarion and Magnus Heresy. Blame it all on one guy.

3. All the other fallen Loyalist Primarchs' deaths can be chalked up to battlefield deaths without even mentioning Chaos. This is also partly helped by the fact that some are missing and in one case a dead one is considered missing by his sons. We 40K players might feel this is weird because we know the true extent of a Primarch's abilities (and that they're very hard to kill) but there would be no contradiction in the mind of the average Imperial citizen. Heroes die and saints are enshrined. That's normal.
- Guilliman fell in battle due to treachery. He WAS actually poisoned, so they don't even have to hide that fact. His body is preserved and on display, so no need to even make up stories about what happened to him.
- Dorn also fell in battle and his mummified fists are on display.
- Khan, Corax, and Russ went away and will return one day. If you don't believe this, the Scars, Ravens, and Wolves would like to have a chat with you.
- Vulkan is missing to the Salamanders. Well, WE know he's basically dead for real (and possibly being kept in Trazyn's vault), but don't tell them that. The Salamanders and the Promethean Cult think he's returning after all his relics are found. Due to the Salamanders being a depleted Legion after HH and their minor presence in the common Imperial citizenry's knowledge base, probably only Prometheans and a few even care that there was a Primarch known as Vulkan anyway.
- The Lion is on vacation. If you disagree, the Unforgiven's tea corner is always open for consultation.
- Ferrus Manus is dead, but his Iron Hands will turn you into a servitor if you tried to tell them that. Due to their stubborn insistence, most likely the mythology around him is fethed up beyond belief by the 41st Millennium anyway.

In conclusion , it's entirely possible for commoners throughout the Imperium to know about, discuss, and even pray for intervention from the Primarchs without Chaos or the Traitor Primarchs ever entering into their knowledge even once.

This is all of the awesome. You pretty much explain the reality of it all: Everybody knows about the Heresy (as a legendary, almost mythical event), but not everybody knows about the Chaos. I'm exalting you so hard right now.

It's been a while since I read Lord of the Night, but who and what was the (rogue?) Witch character that recognises the NL Chapter Symbol on Zsos shoulder pads?

There's so many different Imperial worlds (i.e. about a million or more) that things will be different on every world. There can be no standard definition. There is only speculation.

My understanding is that most of the records about traitor legions are expunged, and knowledge about Chaos is treated as heretical and people with it are dealt with by the Inquisition. So I would assume knowledge of Chaos is heresay in general, and mostly shared in backrooms throughout the Imperium.