The False Heresy
The Emperor was clearly surprised by the Horus Heresy. He had JUST entrusted Horus to the role of Warmaster, he had embarked on a massive project that would require nearly all of his attention. His key components for this were Horus, as warmaster, taking the mantle of leadership for his military operations, and Magnus, who would be filling in the Golden Throne running the Astronomicon and helping protect the webway gate beneath the throne.
Let’s rewind a bit. The Emperor struggled greatly to bring the Primarch project to fruition. He simply didn’t have all the tools and knowledge he needed to bring forth 20 beings so mighty as to enable him to carry out his plans for mankind. He simply couldn’t bring forth the generals who would lead, and help create (in every sense) the armies that would carry out this great campaign. So he does what anyone would do when work piles up and you never make any headway. He took a vacation. Specifically he headed to Molech. A minor world with a neat little gate that made for the perfect tourist attraction.
During his visit the Emperor did what any reasonable tourist would do when stressed about work. He made a deal with the Dark Gods he planned to destroy to enable him to finish his work project. What hasn’t been stated, and can only be guessed at, is what exactly the bargain was, specifically, what the Emperor’s payment would be. I’m going to place a bit of a wager here and say that the bargain was twofold. First the creation would be imperfect, the beings so created would be scattered across the galaxy, and only brought back only with great effort and loss. The real doozy though? The second part. I’m going to hazard a guess that the wording was along the lines of “Some” of the primarchs being lost forever. If you look at the abilities of the Primarchs they had a lot of overlap. There’s artisans and generals, tacticians and strategists, empire builders and savage destroyers. The Emperor clearly designed them with losing “some” in mind. During the great Crusade, sometime before Corvus Corax and Alpharius were rescued, two primarchs and their legions disappeared.
The lack of information regarding the pair is so scant as to call it “reading tea leaves” is an insult to the accuracy of tea leaf readers everywhere. I’m choosing here to read a lot into the phrase “The forgotten and the purged” so let’s take a moment and do that. By referring to The forgotten and The purged (emphasis mine) it seems to indicate that one has been forgotten, and one deliberately purged. I think that gives some indication of how they might have been lost. Being Forgotten doesn’t seem a terribly harsh punishment. Harsh, yes, but I think it represents a failure not of judgement, but something in their basic makeup. A mutation so atrocious that they could no longer be tolerated. Sanguinius alludes to this when talking to Horus and stating that if their “Red Thirst” were to become common knowledge, there would be a third missing statue at the walls of Terra.
Purging seems to hint at something greater. Something so far from the orthodoxy that it was hunted down and exterminated with prejudice. Leman Russ talks about Astartes fighting Astartes before the Horus Heresy and I believe he shows more than a little relish in it. This and references to being the “Emperor’s executioners” and his actions in dispatching squads to keep an eye on all the Primarchs during the opening stages of the Horus Heresy (including some that had already turned traitor… oops) hints that he had more than a little involvement in the purging. Purging also implies removing a taint. I think that this taint was the taint of chaos. I believe that one legion went beyond the arcane learning of the Thousand Sons and become so lost to warpcraft that they had to be destroyed as chaos worshippers.
So imagine this, you are the Emperor. You have created your Primarchs, completing the Dark Gods portion of the payment. They were scattered during your creation of them, and during your reclamation of your instruments, “some” were lost, one to incidental corruption, one to more diabolical corruption. Great! Now that that’s all behind us let’s deal with the biggest threat to stability (Orks) and make with the Empire Building! My trusty son Horus will be of great help in seeing that all this comes to fruition!