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8 Primarchs turned traitors and joined Horus in his heresy against the Emperor and his crusade.

The simple, global reason is that they were closer to the Warmaster than the Emperor and when Horus turned against his father, they decide to join him out of loyalty. I don't completely buy it. Sure, loyalty and trust in Horus is part of the explanation, but Primarchs are depicted as extremely intelligent, with a high sense of strategy, tactics and logistic, and couldn't ignore that a rebellion would lead to a violent civil war that turns brothers against brothers and turn the Imperium into a wasteland. But many of them joined Horus without a second thought and seems to have no problem with slaying other Space Marines during Isstvan events.

The reasons for the treasons of some Primarchs has been explicitly given in the lore. For example, Lorgar believed that mankind needs faith and was disappointed by the fact that the Emperor refused to be considered as a god. His bitterness grew after the events on Monarch. He instigated the Horus Heresy in the name of the Chaos Gods. Angron hated the Emperor since they met, and Alpharius/Omegon decided to join the Heresy to fulfill the plans of the Cabal.

Primarchs must have reasons to join the Horus Heresy. What are the individual causes (personal reasons, or reasoning) that pushed each Primarch to join the Heresy?

The case of Magnus eludes me. At first, because of the events with the Golden Throne and the subsequent events on Prospero, I believed that Magnus' fall to Chaos was independent of the Horus Heresy. At the beginning of the raid of the Space Wolves on Prospero, Magnus seemed to regret his actions and it is not until Arhiman pushed him to action that he decided to save his children and flee to the Sorceror's Planet. To my surprise, Magnus appears at the end of Slaves to Darkness and shows total allegiance to Horus. He can certainly not come back to the Imperium but I don't really understand why he would follow Horus in the destruction of the Imperium. He knew that Horus' rebellion would lead to a catastrophe for the Imperium and mankind, since he tried to warn his father of the treachery of the Warmaster. He certainly knew that one cannot deal with the powers in the Warp without paying a high price.

Note: I read about Magnus' shards but I don't really understand this part of the lore.

Why would Magnus decide to join Horus in his Heresy?

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He did not really. He was tricked by Tzeentch to do so. From Prospero Burns:

‘The Crimson King was loyal. Misguided, but loyal. So this tragedy need never have happened?’

He couldn't save his legion unless he turned to chaos, which was Tzeentch plan.

He has seen Horus turn against the Emperor and wanted to warn first Horus and then the Emperor. The problem he had: By conventional means, he would've been too late to warn the Emperor plus he wanted to prove that psionics is useful. So, he sent his mind to Terra in order to warn the Emperor but was held back by a psionic wall erected by The Emperor to protect Terra and his Webway project. He couldn't pass but got help by a, as Magnus thought, benevolent Warp Entity (Tzeentch). With this, he broke the barrier and allowed daemons to flow into the Imperial Palace. From the Lexicanum article on Magnus:

Magnus returned to Prospero, intent on pursuing his sorcerous experiments in secrecy. He peered into the Warp, and saw a vision of Horus' revolt and roles all the legions would play, except his own. Entering the mind of his brother while it was under a Chaos ritual initiated by Erebus and Cultists on Davin, Magnus attempted to persuade his brother away from heresy and remain loyal to their father.[2] When that failed, Magnus decided to warn the Emperor via an astral projection spell, because a Warp storm was blocking astropathic messages to Terra. As his astral form blazed through the Warp, he came across a Webway corridor that led to Terra and decided to take a shortcut through it. Magnus did not know that this particular corridor was built by the Emperor, and was part of his top secret Webway project. Magnus tried in vain to breach the wall of the corridor, but then an anonymous voice from within the Warp offered Magnus the extra power he needed, and the overconfident Magnus accepted without question. Magnus tore a breach in the wall and followed the corridor to Terra, bursting through the portal beneath the Golden Throne. The breach allowed daemons to invade the Webway and ruin the Emperor's Webway Project. Enraged, the Emperor did not listen to Magnus' warning and banished him from his presence. The breach inside the Webway went on to become known as Magnus' Folly.

The Emperor ordered Leman Russ to bring Magnus to answer for this. The communication was then changed by Horus to exterminate the Thousand Sons, which a Strike Force of Space Wolves, Custodes and Sisters of Silence did. Russ fought Magnus and killed him, but Magnus got resurrected as a daemon prince of Tzeentch (From Prospero Burns)

WHEN WE EMERGED, the battle was done. The Wolf King had engaged Magnus in monumental single combat, and broken his spine. Then, at the very moment when we bested the daemon in the temple hall, sorcery boiled loose across the entire, ruined world. Blood rain fell. The Crimson King, and those of his Thousand Sons who had survived, vanished, fleeing by means of their proscribed magic. Only in this way could they escape total extermination by the Rout. Let this lesson be remembered.

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    One Shard of Magnus became the Demon Prince, his noble side the part that would have still had him refuse to support Horus was separated out and became part of the Grey Knight called Janus, the First Grandmaster. The importance of that should not be ignored because, had Magnus not split into shards, he would likely have at worst sat back and not got involved in the Heresy, or at best, despite the attack by Russ, still tried to support and protect his father. – Richard C Nov 11 at 10:49
  • Awesome answer. I loved reading Thousand Sons but for some reason, I delayed reading Prospero Burns. I believed that the teleportation of the Thousand Sons to the Sorceror's Planet and Magnus becoming pure energy was all a demonstration of Magnus' immense powers. So, the Magnus we see at the end of Slaves to Darkness is just the Daemon shard? – Taladris Nov 11 at 12:27
  • Yes the Khan met a shard of magnus on Prospero who explained what happened, basically his soul was shattered into shards with each shard representing a different aspect or part of Magnus. His Nobility and general goodness formed one shard but that was not part of the whole that formed the being that we know as magnus the red. – Richard C Nov 11 at 12:42
  • we actually don't know for certain it was tzeench that helped magnus in the warp, it is only heavily implied but not verified. @RichardC Multiple parts of magnus became a deamon prince when he ordered them recollected, only 1 of them(as you said noble one) was not returned and became infused with Arvida. – TerranGaming Nov 11 at 16:05
  • @TerranGaming there are many other shards of magnus out there, there is a series of novels about Ahriman trying to track them down individually and find them set long after the heresy. They are also mentioned in other novels. – Richard C Nov 12 at 11:13
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Magnus did not join with Horus, the Crimson King did, this was a being that contained most of the essence of Magnus but not all. Ahriman gathered most of the shards and used them to reform his father into the Crimson King, the being we now know as Magnus, this being was a shadow of his true former self. Magnus's first act was to declare that he would join Horus' rebellion and lay siege to Terra to reclaim his greatest fragment (Janus).

This is told in the Crimson King book

So in answer to the question "Why did Magnus join the Horus Heresy" Because he needed to get his shard from earth.

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I think you're totally right when you say:

I believed that Magnus' fall to Chaos was independent of the Horus Heresy.

He fell to Chaos not because he wanted to join Horus, but because of everything that happened and once he fell, he was totally consumed by Chaos. So what we see now is not the Magnus from the Great Crusade, but a completely different entity: Magnus the Daemon Primarch.

And so, his motivations and actions are not those of Magnus the Red, but of this new demon.

  • It was not independent to the heresy, it was a key part instigated by the powers of Tzeench. In fact at one point Tzeench tells Magnus they originally saw him being the champion, having the role that was now Horus's. No heresy, no opportunity for magnus to break Nikia and allow Horus to tell Russ to destroy him. – Richard C Nov 11 at 12:44

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