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Warhammer 40K: Five “Other” Important Horus Heresy Battles

7 Minute Read
Sep 7 2020

The Horus Heresy is packed full of fantastic “Other” Battles – let’s shine some light on these other important battles that changed the tides of history.

If you frequent BoLS and are a fan of the Horus Heresy, I could probably ask you to list off some of the most famous battles of the Horus Heresy and they would probably look something like this:

Isstvan III

Isstvan V (Drop Site Massacre)

Battle of Calth

Battle of Prospero

Battle of Beta-Garmon

Siege of Terra

If you ask any Horus Heresy player those are the ones that are most likely to come-up as the important ones. But seeing how today is our own Memorial Day here in the real world, I figure it might be a good time to take a look as some of the important battles of the Heresy-Era. While these battles aren’t as flashy, they were just as important and helped to set the stage for the future of 40k.


Battle of Diamat


I know a lot of folks like to give the Dark Angels a hard time for their “participation” in the Horus Heresy. There are lots of signs that point to the fact that perhaps they were just trying to wait and see who was going to win before they jumped in. While I do think there might be some merit to that theory, we’ll never know for sure (unless GW come right out and confirms that). But that’s one of the reasons I propose the Battle of Diamat as evidence that the Dark Angels weren’t all bad.

Shortly after the events of Isstvan III, the Dark Angels got word of the betrayal. But, because they were already engaged in a campaign (more like setup by Horus to be occupied) they couldn’t commit their full fleet to supporting the Emperor. They sent a small force to assist that was going to target the Forge World of Diamat.

The long and short of it is that the Dark Angels overcame some pretty staggering odds and defeated the Traitors. However, the Iron Warriors then tricked them by acting loyal to the Emperor. Lion El’Johnson then provided the Iron Warriors with the remaining siege engines because he believed they would be more effective in their hands. And they were…at the Battle of Isstvan V. Oops. Perhaps this is what caused the Dark Angels to “sit out” for so long. Many they just weren’t sure who to trust – that and they had their own issues during the Heresy.

Chondax Campaign


The Chondax Campaign took place towards the end of the Great Crusade and bled over into the Horus Heresy. The White Scars were battling against ork and had recently defeated them in the sector when an Alpha Legion fleet arrived and basically held them hostage.

There was a tense stand-off, some fleet-based heroics and a lot of confusion sown. Sounds like Alpha Legion’s motive, right? However, part of the reason the White Scars were confused is that they received intel from both Horus and Russ claiming the other had betrayed the Emperor. This led them on a course to Prospero…but that’s not all:


Later Mortarion remarks that that something foul had taken place at Chondax, as the original plan by Horus was for the Alpha Legion to recruit the White Scars. Instead, either under orders from Alpharius or Omegon, they took overtly hostile actions, avoided any attempt at communication, and kept the White Scars occupied until they could recieve a message from Rogal Dorn warning them of the Heresy. Indeed, the only reason this message could be delivered was because Omegon had dispatched his Effrit Stealth Squad to destroy a Pylon blocking White Scars communications. It seems that the Alpha Legion, or at least elements of it, wished for the Scars to remain true to the Emperor.

Hmm…that seems pretty odd, don’t ya think? That’s why this battle is important. It sets up the White Scars to follow-up on Prospero and it casts some doubt on where the Alpha Legion’s loyalties lie…but they are Alpha Legion, after-all!

Battle of the Alaxxes Nebula


Let’s set the stage: It’s shortly after the Battle of Prospero. The Space Wolves are reeling from the massive losses they (unexpectedly) took during the fighting. Suddenly another Fleet shows up and it’s those pesky Alpha Legion Marines – but this time, they don’t mess around. They open fire and they attack which forces Russ to order an immediate retreat into the Alaxxes Nebula. This is when Russ sends out a distress call to none other than Jaghatai Khan (see above). Khan decides not to aid Russ but instead orders his fleet to Prospero.

The Wolves are completely surrounded and it’s looking bad. Russ decides to throw a pity-party and locks himself in his private room. This leads to some self-reflection with the aid of Bjorn. But all the while they are still being attacked and First Captain Gunnar Gunnhilt takes command. The Captain decides to attempt a breakout maneuver instead of resigning himself to die like his Primarch had. So of course Russ shows up at the climax of the fight and takes command with a renewed sense of purpose!

Long story short, Russ squares off with possibly Alpharius in disguised (because Alpha Legion) and they get bailed out by none other than the Dark Angels. The Dark Angels who bailed him out were actually Luther Loyalists, by the way, who just wanted to figure out who was the “real” traitor to the Emperor. In the Aftermath, The Dark Angels head back to Caliban (see “own issues” above) and Russ sets course for Terra!

Why is this important? First – it shows that Leman Russ, a Primarch, isn’t so far removed from humanity that he can’t feel hopeless. It’s kind of a humanizing aspect in a depressive sort of way…but more importantly, I think this battle is important because HOLY COW – what a twist!? I almost feel like this was an M. Night movie script or something. But I guess I just spoiled all that now. Sorry.

First Siege of Hydra Cordatus


Hydra Cordatus is an Imperial Fist planet. It was liberated by them and they garrisoned the world. So, naturally, the Iron Warriors and Perturabo are the ones that are going to smash this place and burn the ashes. One of the cool things about this place is the Cadmean Citadel:


No one knew who built the Cadmean Citadel, and the fortress had the ability to automatically repair damage that was inflicted to its structure by pouring liquid silicates that would fill any wounds. The wall had other defenses including the ability to form spikes that would pierce any invaders or it would open gaps and cover an attacker. Beneath the wall were gravity generators that slowed anyone would who attempt to climb, but their range was limited to only a few meters. The Mechanicum performed extensive research on the facility but could not understand how it worked. The fortress was not invincible; it could be damaged to an extent in a short period of time that made it irreparable, overwhelming its mechanics. Neither the material nor the technology came from Hydra Cordatus.

The siege itself actually takes 3 months to complete and Perturabo himself leads the final assault. I’m glossing over the details at this point because I’m betting you figured out what happens.

So why is this battle important? Well if you’re familiar with the history between the Iron Warriors and the Imperial Fists, you’ll understand. This battle is a foreshadowing of what will happen on Terra. The Iron Warriors and their siege breaking abilities will be put to the test against the Imperial Palace. Had the Emperor himself not assaulted Horus’ Flag ship things may have gone very differently…

Battle of Molech


The Battle of Molech took place on an apparently insignificant world. Horus led an assault on the planet because of an old memory that had somehow been unlocked. He remembered a Warp gateway that the Emperor used to gain power was there and he wanted to obtain the power himself. In the end the Traitors achieved victory and Horus was apparently successful in gaining whatever power he sought there.

This was an important battle because of the sheer size of the forces that Horus overcame:


That’s no joke. On top of that, Horus got whatever power he was after – so not only did he successfully defeat a massive Imperial force he also became more powerful in doing so. Considering this was more than just a random pit-stop on the way to Terra, I think that qualifies it as a pretty important battle.


There are LOTS more battles that have been written about for the Horus Heresy and it doesn’t look like the Black Library is slowing down anytime soon. I’m sure there will be even more fantastic “other” battles to read about.

If you want to learn more go check the Lexicanum



So many battles…so little time…

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