Did any members of Loyalist Legions turn traitor during the Horus Heresy?

Question

There are plenty of examples of Astartes in Traitor Legions remaining loyal (Garviel Loken, Garro, Saul Tarvitz, etc.), but are there any instances of the opposite happening? Excluding the ambiguous case of the Dark Angels, did any Astartes from Loyalist Legions fight for Horus?

Answer 1

Yes there have been some cases of marines of Loyalist legions that swapped sides, willingly and unwillingly, or even for only a certain amount of time.

Captain Alastor Rushal of the Raven Claw legion.

In the novella "Prince of Crows" Captain Alastor Rushal from the Raven Claws. Who joined the Night Lords after the events of the drop site massacre on Isstvan 5. It is unknown however if he did so willingly or not. This excerpt from Prince of crows tells that he was a Captain of the Raven Claws before joining the Night Lords and scratching away the marks and disgracing the markings to his former allegiance. (Emphasis mine)

Last of all was Alastor Rushal, born of Terra, but not born of VIII Legion genestock. He still wore the armour of his Legion, cast in a cold black, edged in dented white trimmings. The noble emblem on his shoulder guard – a raven in white, with wings spread wide – had been ritually broken by blows from a hammer, wielded in Alastor’s own hand. All trappings of rank were gone from his armour, scratched away after the killing fields of Isstvan.

A part of the White Scars

During the Second battle of Prospero a group of White Scars Legionnaires from a warrior lodge within the legion led by Hasik Noyan-Khan and Torghun Khan where they attempted to lead their group of heretical white scars to take control of the White Scars' fleet and join the Death Guard led by Mortarion at the time while Jaghatai Khan was on Prospero at that time. The plan failed however when Shiban Khan managed to board the White Scars flag ship, Swordstorm.

From Scars by Chris Wraight (Emphasis mine)

‘Brothers, Hasik Noyan-Khan has control of the Swordstorm. From there he controls the Legion in the Khagan’s absence. He must not be allowed to make the decision for us. That is why I have called you here. It means assuming the mantle of renegades, at least in the eyes of those who now seek to subvert us. It means taking up arms against our own brothers. You do not need me to tell you that no such rebellion has ever occurred inside the White Scars. We risk our honour, and may pay for it with our lives.’

Shiban clutched the hilt of his glaive tightly. ‘I cannot demand this of you. We will not be fighting xenos – these are our own people. All I can do is ask you to trust me. I have led you across the arc of the galaxy in the cause of the Great Crusade. We have brought compliance to hundreds of worlds and given honour to the name “White Scar”. You followed me then. Brothers, you have heard what I judge to be true.’

He paused for a heartbeat. ‘Will you follow me now?’

Shortly after Jaghatai Khan returned to a blood bath between the loyal white scars and the traitorous white scars.

The Khan strode down from the platform, his keshig following him closely. Ahead of him, the command hall remained swamped in combat. Many of those close enough to the teleportation flare to hear it over the clamour of the fighting broke off in sudden confusion, but others remained committed, locked in the storm of bolt-shells that crisscrossed the entire space. For a terrible moment, the Khan witnessed the warriors of his Legion at each other’s throats. Mortarion’s words rang in his head, as mocking as that final salute. Half your Legion are already declared for Horus. He scanned over to the command throne. The fighting was heaviest there. With a lurch of recognition, he saw Hasik occupying the dais, fighting hard to repel a surge from Jemulan’s warriors.

When he arrived MOST of the White Scars surrendered.

Apothecary Meros of the Blood Angels

Whilst this case was "willingly" it certainly was not because he wanted to join Chaos but because he wanted to save his primarch Sanguinius from that fate. But I would still like to include this.

From Fear to Thread by James Swallow (Emphasis mine).

During the Battle of Signus Prime keeper of secrets Kyriss the Perverse tried to persuade Sanguinius to join Chaos by promising that he would make sure the Blood Angels legion would be fully cured if Sanguinius would submit to the Ragefire, Sanguinius was willing to accept this offer and sacrifice himself willingly for the sake of his sons but when he was about to step into it Meros did this before him and Sanguinius follow quickly behind Meros into the ragefire.

Without hesitation, Sanguinius threw himself into the air in a flash of white and gold, soaring upwards in a spiral after his errant son. He committed the act without uncertainty. He knew it was right. If there had been time to doubt, Meros might have wondered after such abstracts as fate or destiny, but he was not one to think in those terms.

Inside the vortex, Meros refused to let his Primarch take the deal.

A heavy rush of air and the rumble of beating wings sounded a new arrival. Meros turned and suddenly there was the stern face of the Angel, alighting behind him. ‘Stand down, my son,’ he said. ‘I command you.’ Meros took a breath, and then he spoke the most difficult words he had ever uttered. ‘No, my lord. I must respectfully refuse.’ Sanguinius’s eyes narrowed. ‘You disobey your primarch.’ ‘Aye.’ From nowhere, a strange mutter of sentiment rose through the Apothecary and he gave a rueful laugh. ‘I suppose that makes me a traitor.’ ‘Meros. You cannot do this.’ The Angel’s wings folded close and he pointed toward the writhing mist. ‘No mortal soul can survive contact with such a force. If it is what the beast Kyriss said, it is the raw force of the warp. It is the crude power of all our rages. You will not be able to control it. It will destroy you.’

Instead Meros cut out his own geneseed and offered it to Sanguinius before allowing the rage to possess him. Meros turnid into the Red Angel. The demon consuming every part of Meros except a small sliver he could still torment.

Meros transformed, writhing in the grip of an infernal glow as the burning power merged with every atom of his being. The flesh of his face distorted into a hollow mask far beyond the guise of pain. His armour became dark and disfigured, joints steaming, the shell of it trembling as it fought to contain energies never meant to be bound to this reality. In the warp’s shadow, Kano saw a pair of ghostly, blood-drenched wings briefly unfold from the wounded Apothecary’s back, anointing the ruined bones beneath with a rain of phantasmal blood; then they were gone. Brother Meros ended, and the Red Angel began.

Answer 2

Not sure it worth an answer but it is too long for a comment.

To my knowledge, Luther and the Dark Angels that were left on Caliban is the only known example of Space Marines that betrayed a loyalist legion during Horus Heresy. That's the biggest shame of the Dark Angels and they'll go very far to keep it secret.

In addition, we have some more information in recent books:

1. In Delivrance Lost, Corax, after the massacre of Isstvan V, comes back to Earth to ask the Emperor's help to rebuild his shattered legion. When arriving to Earth, his legion is put into custody the time their loyalty is confirmed.
2. In the novela about Lion'El Jonson in The Primarchs, the Dark Angels decided to attack a planet where the Death Guard is fighting against a force of survivors of Iron Hands. Instead of siding with the IH, the Lion decided to treat each faction as potentially suspicious, arguing that in this time of crisis, it cannot be certain that the Primarch-less Iron Hands had all stayed loyal to the Empire.

So we can guess that loyalty of the legions was put into great scrutinity in the aftermaths of Isstvan V. My understanding of the situation is that the massacre of Isstvan was such an horrible event for every loyal Space Marine that all considered that Horus had gone too far: coldly setting a trap to kill thousands of Space Marines, including some of his brothers. All books that relate the events after the massacre show how shocking this massacre was for Space Marines and probably reinforce the loyalty to their legion.

PS: it should also be noted that Space Marines turned traitors after the Horus Heresy, e.g. Darkheart Huron.

Answer 3

I've never read of any loyalists jumping ship after the Heresy began.

Prior to the trap which led to the Istavaan III massacre, Horus recruited every Primarch to his side that he possibly could.

That was the whole point of that trap, to kill everyone he knew would never join him. If Horus couldn't sway a Primarch to his cause, that Primarch's Astartes would certainly not join him.

Once the Heresy began, Horus was planning on wiping out every single loyalist marine in the galaxy in one fell swoop. Coupled with the events of Istavaan III, he sent the Word Bearers to ambush / exterminate the Ultramarines and I'm not positive about the White Scars but I'm sure he had plans for them too.

Answer 4

Aside from the Dark Angels thing, with there being doubts just which side turned to Chaos, no known members of the nine loyalist Legions turned to Chaos.

Answer 5

Well there were a lot of expedition fleets. And in those fleets there were elements of many chapters thet were far away from their legoins/Primarch. It could have been relativly easy for Horus and the traitors to develop some kind of a warrior bond with some of theese Guys. And in so doing it would have been simple to start undermining the primarchs.

like why are you here and not by your primarchs side? Arent you good enough? is your egion so ashamed of you that they send you away? and so on...

let that simmer for a bit and then poke at it some more...

Result... yes, you guessed it. A possability of a "loyal" legionare turning traitor.

Answer 6

Horus Heresy book 4 Conquest states that
"nearly every legion had marines fighting on both sides or even neither side of the Heresy. Iron Hands, Raven Guard, Space Wolves, and White Scars all have listed examples of marines who either joined with Horus, or went off on their own."

there was a Great Company of Space Wolves bearing the Serpents Eye of Horus, who slaughtered millions at Neo Cadiz in 008.M31 as well as Iron Hands who wore a dual Eye of Horus/cog heraldry and who fought at the Siege of Mezoa. There were also Terran Raven Guard who greatly respected Horus above Corax and had begun to join the Word Bearer lodges. While the vast majority of these marines died during the Battle of Gate Forty-Two, there could still be a small smattering of them either amongst the legion or who fled at Istvaan. The FW books make it clear that these are only the most well known cases and they plenty of others existed across all legions, in both directions.