The War of the False Primarch

War of the False Primarch
Date	From 780.M33 to 860.M33
Scale	Unknown
Theatre	Segmentum Pacificus
Status	Decisive Imperial Victory
Belligerents
The Pentarchy of Blood	Renegade and heretical Space Marine Chapters
Commanders and Leaders
Chapter Masters of the Pentarchy, High Lords of Terra	Unknown
Strength
Carcharodons, Red Talons, Flesh Eaters, Charnel Guard, Death Eagles, and presumably Inquisitorial allies and Imperial Guard reinforcements.	Unknown, but at least 11 traitor Space Marine Chapters and an unknown amount of human rebels.
Losses
Unknown	Complete annihilation of the traitor chapters and their homeworlds.
Outcome
The anarchy in the Segmentum Pacificus is ended as Imperial order is restored. Traitor chapters and their forces destroyed in totality to the point that they are not remembered or recorded in Imperial history.

First described in the second edition of Imperial Armour Volume II, The War of the False Primarch was a dark and bloody episode of the Imperium's history, now largely lost to myth and purged from all records, that plunged the Segmentum Pacificus into anarchy from 780.M33 to 860.M33.

How this whole fuck-fest started is completely unknown, which is bullshit considering that this is allegedly an event that is AT A MINIMUM comparable to the Badab War.

The war was ended when the High Lords of Terra convened the Pentarchy of Blood - a gathering of five Space Marine Chapters (specifically, the Carcharodons, Red Talons, Flesh Eaters, Charnel Guard, and Death Eagles) tasked with destroying eleven other Chapters that had been declared Traitoris Perdita... for some reason. The text suggests that eleven "traitor" chapters somehow went too far during the pacification efforts of Segmentum Pacificus, but the text neglects to mention whether or not the chapters themselves were responsible for the anarchy in the Segmentum, or were merely too enthusiastic in their purging of the rebels for even their fellow Catholic Space Nazis. We don't even know if the civil conflict in the Segmentum is part of the War of the False Primarch or if only the battles involving the Pentarchy are the War itself. And above all else, we don't know whether or not this was merely lowercase-h heresy (mundane rebellion), or capital-H Heresy (Chaos involvement).

Either way there doesn't seem to be a compelling reason as to why the war was "soooo sekrit and terrible" aside from GW and Forge World's incompetence and faggotry fueled by their need to sell the expensive hunks of plastic and resin in their Imperial Armour books. "Lore? Fuck you nerd, do our jobs and write it yourself."

Speculation

Challenge accepted. For such a large scale conflict, the fact there isn't any substantial fluff regarding it is a travesty. Playing Devil's advocate and presuming that there is an actual reason for why there is no real lore for this conflict aside from the greed and incompetence of the Eternal Anglo, we can theorize several reasons as to why in-universe this war has been censored from Imperial records.

The biggest hint as to what might have happen is undoubtedly in the name of the conflict itself. What is a False Primarch? Was it literally an impersonation of another Primarch or similarly revered figure that led the eleven traitor chapters astray, or did an actual Primarch somehow come back to Imperial space to lead a rebellion? In the case of the former, Alpha Legion fuckery is understandable as to why there are few concrete records of this event, but does not sufficiently explain the active purging of the archives. In the case of the latter, the active censorship and secret-keeping makes a lot more sense if this rebellion was masterminded by the return of one of the lost Primarchs. Also, the purging of the knowledge of the identities of the eleven Chapters further encourages this idea. That could have been actually a legion (the legions had Chapters aka Great Companies, after all) made by a returned forgotten Primarch. Perhaps the eleventh, with eleven chapters being a hint for the audience. Could Fabius Bile have had something to do with it? After all, he has successfully cloned at least three Primarchs- albeit only one of them had the original Warp-imbued soul of a Primarch. Whatever it was that the Lost Legions did was apparently so potentially damaging to the unity of the Imperium itself; something somehow even worse than Horus's Heresy; that active censorship and maintaining the edict of Damnatio Memoriae on them even after the Emperor has been interned on the Golden Throne and Malcador perished is only logical. Just what could be so bad, is something that HOPEFULLY the writers of BL will get around to and not fuck up.

With that hypothesis established, the brutality and totality with which all involved traitors were dealt with makes sense, and is reinforced by the choices of chapters that were selected to carry out the purges. The Carcharodons, Red Talons, Flesh Eaters, and Charnel Guard could be counted upon to kill everyone involved without mercy or concern for collateral damage, something necessary in what was likely a very dirty war. The Death Eagles however appear to be the odd one out; lacking the brutal reputation of the others; but their inclusion in the Pentarchy of RIP AND TEAR could be the likely result of the High Lords blackmailing them over their origins into providing support and retrieval of whatever secrets that this war unearthed. On the other-hand, with their obsessive fatalism they probably aren't at all picky about who they kill since everyone is inherently doomed from birth anyway.

The timing of this event may also provide reasons as to why it was covered up. Happening in the last quarter of mid-M33, this even is sandwiched between The Beheading and the Nova Terra Interregnum, both of which were immensely disruptive to the unity of the Imperium. After the Beheading, the High Lords were probably scared shitless over possible threats from within, the last threat had seen both the infighting of Imperial factions reach an unacceptable level and their predecessors killed and the Imperium ruled by a madman until the Space Marines were able to restore the more-or-less rightful order of things. Things were in still in relative anarchy due to the fallout of the War of The Beast and the Beheading, however, with only the Space Marines not being affected by this factionalism and infighting. At this time, the worst case scenario for the unity of the Imperium would be something that would directly affect the loyalties of the Space Marines themselves in these turbulent times, as they were the most solid rocks in the storm of civil strife that afflicted the Imperium. Whatever the False Primarch was (if they even literally existed and were not a figure of speech), they would logically be that worst case scenario, as whatever happened led to eleven chapters and a whole Segmentum going rogue, and their legacy would leave indelible scars that the wounded Imperium could not afford to publicly bear at this time. Thus, memory-holing the problem was the only rational decision to make if something so drastically threatened the loyalties of the Space Marines.

With the Beast Arises series finished, the next time period in line for the Black Library authors to cover would be either this one, or the Nova Terra Interregnum. Both are criminally lacking in details, and thus it is only logical that they be covered next in the fleshing out of the Imperium's past. Fingers crossed!

This is NOT a stub, that's all there is to know.