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The Hours Heresy: The Siege of Cthonia

Greetings one and all,

I've just finished reading over The Siege of Cthonia, so I thought that this week I'd write a short piece on my impressions of the book, its contents, and if it's a worthy addition to the current edition of the Horus Heresy. So, without further ado let's get into it.

What is the Siege of Cthonia?

The Siege of Cthonia is a 240-page hardback book and much like every other book in the Horus Heresy setting so far, it’s full of new lore, additional rules, tons of details and pictures on nearly every page.

The book covers The Siege of Cthonia which is under Imperial Fists occupation. The story is that of a minor faction within the Son of Horus, the True Sons, who have broken away from the march on Terra and are now trying to retake their homeworld and the chaos and carnage that results.

Beyond that, The Siege of Cthonia contains 5 more core missions, two Apex missions and an entire section on Zone Mortalis games, including more new missions specifically for the Zone Mortalis game type.

Lastly, there are new rules for both the Imperial Fists and the Sons of Horus, Legiones Astartes: Decurions, Cult Abominatio and Inductii.


As to be expected, there is a huge chunk of lore in this book with a full 130 pages of new background material. And not all of it is on The Siege of Cthonia either, with the Sieges of Baal, Inwit, and Barrarus all getting a mention. Even unheard-of Planets like Foricaan get nearly 3 pages of lore really adding to the scale and depth of the Horus Heresy.

But the main bulk of this new lore is about the book's namesake, The Siege of Cthonia, and here the level of background material really shines. If you were a fan of the Black Forgeworld books or the Imperial Armour books like Siege of Vraks or Badab War, then this will be a great treat for you.

I won't go into much detail as I don’t want to spoil anything too much for anyone, but it’s got everything; space battles, drop assaults, a battle that twists and turns and then turns again. The Sons of Horus under Cadre-Captain Vheren Ashurhasddon want their homeworld back and the leader of the Imperial Fists Lord-Castellan Evander Garrius has been ordered by Dorn himself, to hold Cthonia at all costs. Meanwhile, the Word Wearers are scheming in the background. Their plans are unknown, but they’re opposed by a loyalist group of Thousand Sons.

Furthermore, there is lore covering the Inductii, how they were made, and why. This is another really interesting take on the sheer scale of the Horus Heresy and the lasting impact it had on the Space Marines, even at the genetic level.

Onslaught Campaign

A new way to play native games. Onslaught Campaigns are designed to be two-player affairs (though this can be changed to two teams) each taking the role of a Loyalist or a Traitor. The Campaigns focus on forcing what is called The Decisive Battle; campaigns last between 2 - 6 games. After each game, the winner rolls on the Seize the Moment table, and if they succeed then the Decisive Battle is played. The winner of which wins the Campaign as a whole.

The Onslaught Campaign comes with a host of Stratagems, Core Missions, and rules for injured Characters, as well as Special Achievements so you really can create memorable games. It’s a fun way for friends/players to create a short story-driven Campaign with plenty of rules to help flesh it out and make each Campaign different.

Zone Mortalis

If you’ve ever wanted to have a game where your forces fight through the cramped tunnels of an Underhive or a fortress, or maybe you love the thought of your warriors fighting their way corridor by corridor clearing out a defence station, or even the interior of a spaceship during a boarding action. Well then my friend, these are the rules for you!

The New Zone Mortalis rules have a heavy focus on terrain, with plenty of rules detailing with specific bits. Personally, I like the inclusion that it is assumed that the battlefield has a ceiling and therefore, units equipped with Jump Packs or Indirect Fire Weapons have to play completely differently.

I think the thing that has really impressed me is the Reinforcement Points Cost Chart, forcing you to feed your army onto the board piecemeal, along with the hard limit on Dreadnought-type units: only 1 per 1000 points.

Furthermore, Zone Mortalis come with a range of missions and Reactions that I think are going to make it a blast to play, and I’m really looking forward to it.

New Units and Inductii

Lastly, we get to the new units., There are a few nice upgrades to both the Sons of Horus and Imperial Fists, I really like the idea of the Ferromag Shotgun being an anti-Space Marine Weapon!

There’s also the Legiones Decurion, which represents the ace tank commanders of the Horus Heresy and can be brought as upgrades to your Vehicles.

The Cults Abominatio get introduced with the first rule set (that I’m aware of) for these dark mirrors of the Officio Assassinorum along with a new forge world model.

But the biggest thing of interest to everyone I’ve spoken to though are the Inductii. These mass-produced Legionaries never saw the Great Crusade and, in effect, stand apart from the old guard of the Legion (even if the Inductii make up the bulk of some legions by the time of the Siege of Terra). The Horus heresy has been all they’ve ever known and their rules show that with each legion, they have their own unique Inductii rule set.

And when a say unique, I really do mean it; from the Space Wolves and their Chainswords and Comabt shields to the Iron Hands using Forbidden Augmentations, giving their Inductii a potbelly +1 Weapon and Blastic Skill at the cost of Wounding the unit. They really are an interesting bunch and totally worth trying out in your upcoming games.


So the question I posed at the start was is the Seige of Cthonia a worthy addition to the current edition of the Horus Heresy? I have to say, yes. It adds entirely new ways to play, interesting rules and new units and I really hope Gamesworkshop keep releasing campaign book of this calibre.

Personally, I think if you’re playing the Horus Heresy, this book is a must-have.


In celebration of this amazing book, I'm giving away one copy of the Siege of Cthonia, all you need to do to enter is to retweet/share, comment and follow on any of my social media accounts listed below. I'll announce the winner on Friday 16th June 2023. Good luck every one

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