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The Siege of Cthonia Review – Warhammer The Horus Heresy

At the very end of The Horus Heresy as the Warmaster battled the Emperor Himself for the fate of Terra, another siege had erupted elsewhere in the galaxy as the loyalists who had taken Cthonia faced a brutal assault from the Sons of Horus in order to reclaim their ancestral home.

in Campaigns of the Age of Darkness: The Siege of Cthonia, up for pre order today, we get our first look at the “black books” for the second edition of Warhammer The Horus Heresy with a book filled with lore, narrative campaigns, Zone Mortalis rules and much more! In this review we’ll be checking out the book, looking at what is included within it’s pages and seeing if this is worth picking up to add to your collection.

Massive thanks to Games Workshop for sending us a free early copy to take a look at to review for the site. If you would like to support the site then why not order your copy through our affiliate Element Games and save yourself some money too?

So what exactly is this book? Well, fans of the 1st edition of The Horus Heresy will have fond memories of the “Black Books” that served as narrative supplements that explored various conflicts and battles across the history of the Horus Heresy while also introducing new rules and campaign elements to expand the options that players had available to them to recreate these battles. Like with the Imperial Armour books before them, the Horus Heresy Black Books acted as a sort of “in universe” recording of the events (which also left some elements as ambiguous or subject to bias in the intervening time between the Heresy and the current 40k timeline)

The Campaigns of the Age of Darkness series looks like it sets out to do the same thing, and the first installment The Siege of Cthonia clocks in at almost 250 pages! Impressively, the first 130 pages are dedicated to lore, telling the tale of the two sieges of Cthonia. In an interesting twist, the war on Cthonia takes place at the same time as the Siege of Terra, with the narrative playing up right until the Emperor killing Horus! I believe this is the first recording of that legendary battle within the Heresy books so far, and shows that this series can include elements from any part of the Heresy, not just playing through things chronologically. This is great, as this opens the door for us seeing supplements for late Heresy armies such as the Dark Mechanicum, who are a very different force compared to the forces listed in the Liber Mechanicum.

Like with the Black Books, we get a page framing this as a historical document, and also calling into question the reliability of the narrator, with a box out debating on if this climax of the conflict actually happened at the same moment Horus fell, despite also telling us it is widely accepted it did…

I really like this set up as it opens the door for lore developments while also reminding us that history isn’t necessarily reliable and that things we accept as facts about the Heresy may not be totally accurate.

With narrative being a big part of this book I don’t want to go into spoilers here, as I know a lot of people will want to read this for themselves. What I will say though is that people who have been longing for an Imperial Armour style book will not be disappointed – we get listings of forces and the armies present, detailed maps with force movements and key locations, illustrations of key players from within the battles and an exciting narrative detailing the events of the entire conflict. For those worried about this being very much an Imperial Firsts vs Sons of Horus story you can rest easy, as numerous Legions are involved here, as are the Dark Mechanicum, Sisters of Silence and Imperial Army (All of whom are good contenders for the “new” plastic army releasing later this year) – there’s also a few surprises with some unlikely allies too!

I do recommend not spoiling things and reading this for yourself, especially as we get half of the massive tome dedicated to this story! I have high hopes for this series going forward and revealing new secrets that we have not yet learned about the Horus Heresy

The Onslaught Campaign

The first section of rules material in the book is the new Onslaught Campaign. This is the new campaign system for 2 to 6 players each with 3000 point armies, and can be used to string together games that are not narratively linked, or can be used to play through a full narrative campaign such as the events of the Siege of Cthonia. It looks like this will be the core campaign system going forward with future books having additional rules and plug ins to expand it further.

When playing an Onslaught Campaign the following steps are taken:

First, the players decide which optional rules they want to use (This includes which narrative additions from the campaign supplements they want to use)

Then they separate into two teams – Loyalists and Traitors. In this initial campaign pack the two sides are distinct with no shenanigans and changing allegiance during a campaign – However it does mention that future suppliments may place restrictions on what can be taken by Loyalists and Traitors to represent the historic forces that would be present at those conflicts

Each player then gets to pick four campaign stratagems – these are one use effects that can be used by each player over the course of the campaign. There are 10 core ones as part of the Onslaught Campaign rules, and then 4 that are unique to the Siege of Cthonia

These all have cool effects such as for one game being able to set up the terrain for the battlefield representing you fighting over a prepared environment, or for a single battle having an extra heavy slot, or being able to take a Primarch in a game of any points level. Some are very powerful such as being able to pick who takes the first turn. My favorite is Titan Support which allows you to ignore the points restrictions for Titans, allowing you to take a Reaver in a 3k game for example (As long as all compulsory slots are filled in your list)

I think these are really fun and add an extra element to games – the thing to keep in mind with these is that they are a limited resource and all other players have them too, leading to a game of chicken for when you play them.

With this being a continuing campaign, there are rules to represent characters getting wounded and damaged as the battles continue. Because of this when a character is killed during a game you roll a D6 afterwards to see if there are any long lasting effects such as having reduced wounds and leadership in following games, or even sometimes gaining extra attacks in the next battle due to being hungry for revenge!

The main mechanic of the campaign is “Seize the Moment” rolls. Basically each campaign round each player will face off against someone from the opposing team, and the side that wins the most battles (Or in the case of a draw whichever side wins a roll off) gets to make a “Seize the Moment” roll – this represents the forces of their side gaining enough momentum to enact the final battle that will determine the outcome of the campaign. The wins of each side are compared and a roll is made to see if the moment is seized. with just one win this might be a requirement of a roll of 6+ in order to trigger it, but as the wins increase it gets easier – for example if your side has 4 wins and your opponent 2 then you only need a roll of 2+ to seize! and if you ever get 5 wins you automatically Seize the Moment.

Once one side has Seized the Moment then a final “Decisive Battle” is played – this tends to be a cinematic “Apex mission” in order to give the final game the excitement and high stakes it deserves. People who have been hording their stratagems in order to affect the final game need to watch out, as only the winner of the Seize can use a single strat, and all others are lost – if you haven’t used them yet tough!

In order to make these Seize rolls more exciting there are a number of different achievements that can be unlocked during games that give you a modifier to the dice roll to trigger the final mission – for example if you end the game with double the victory points of your opponent you get a +1 modifier on the dice roll. 4 different achievements are listed, and the book hints that future volumes may have more to come!

We also get a couple of pages of Siege of Cthonia special rules – these are flavorful things that affect the battles in a way that does a decent job of mirroring what happens within the narrative. Again, I don’t want to go into spoilers too much, but there’s things that allow the loyalists to place additional fortifications at the start of the battle, and modifiers that negatively affect psychic tests and positively affect the strength of daemons…

In order to play these campaign missions (and for adding to the pool of available missions for one off games), the book includes 5 Core Missions. Anyone who has played any Horus Heresy events at Warhammer World will recognise these, as they have been in use (through a couple of iterations) at their events!

These all play for 4 turns (which in my opinion is the perfect length of a game of Heresy) and have both victory conditions listed within the mission itself and additional secondaries such as getting the first kill, slaying a warlord or taking down a super heavy!

These missions give us a much needed expansion in the missions available to play, and I hope that these are added to with future campaign books in order to keep things fresh every year in the same way Chapter Approved does for 40k.

The exciting part for narrative gamers however are the Apex Missions – these are very thematic missions based around the events of the narrative in the book, and also act as the closing missions for the campaign. The twist with these is that the players are allowed to read the mission in advance and build their army to the theme of the mission. This is really cool as it allows you to really capture the feeling of the conflict from the lore on the tabletop.

These missions are not necessarily “balanced” for matched play, often with one army being bigger than the other – but that’s the point of this book, it’s to recreate the events of the battle, and sometimes those battles are one sided or swing wildly one way, the fun comes from getting to try your hand at changing the outcome of what “actually” happened during the battle.

For example in one scenario the defender has 25% more points than the attacker. However, the attacker gets to drop mines and even hulks out of orbit onto the battlefield below before their forces make the assault!

On the other hand the second mission uses 4000 point armies and has the defender starting with 2k on the board while they wait for their reinforcements to arrive, while the attacker ambushes them from hidden underground bastions represented by blast markers on the battlefield – the defender can try and destroy these bastions to try and stop the flow of the attackers, however that means less shots into the attacking forces..

The defender has a hard fought battle as the only way they can bring a unit on from reserves is to spend a victory point to do so. And so the players get an awesome last stand where the defender will be fighting to within an inch of their life to gain the resources to bring on more units.

While these two missions wouldn’t work in a matched play setting, they make for some very cool and memorable battles – and I can’t wait to see these expanded upon throughout the series of books!

Zone Mortalis

In addition to the campaign rules, Siege of Cthonia also contains a full Zone Mortalis expansion. This popular game mode allows you to recreate battles that take place in the right confines of fortress tunnels and defense networks. Obviously, compared to a “normal” battle things are a little different when you are fighting through tunnels, forcing your way through doors and moving with limited visibility.

Because of the changes in battlefield environment, a number of rules are introduced to try and capture the feeling of battles like the assault on the Saturnine fault.

Firstly, Zone Mortalis battles are smaller. A 4″ board of Zone Mortalis tiles is used and armies are between 1500 and 2500 points. Army construction is also different with a unique force org chart (1-2 HQ, 1-6 Troops, 0-4 Elite, 0-2 Fast, 0-2 Heavy 0-1 Primarch(and only in games of 2k+) )

The maximum unit size is 15 models, lots of units are forbidden (fortification, vehicle, armiger, cavalry, antigrav) and Dreadnoughts, Monsters and Automata can only be taken if they have less than 8 wounds.

This all ensures that the type of armies taken reflect those that would be present within these environments. As you would expect there are lots of rules dedicated to how terrain works in these games, including some detailed rules on doors and even rules to reflect a door closing on a poor unfortunate victim (Spoilers – it doesn’t end well for the trapped person OR the door!). With Forge World no longer making the resin Zone Mortalis tiles, the book includes guidelines on how much plastic ZM scenery you should use on each plastic tile in order to create a fun and balanced game.

In the tight confines of the Zone Mortalis, no core reactions can be used from the Age of Darkness book – instead 3 new ones are introduced

Suppress – allows a unit to snap fire at a target in the movement phase – all weapons gaining pinning as part of this attack!

Displace – allows a unit to move equal to their initiative when shot at, potentially allowing them to get out of range or out of line of sight

Brace – done when a charge is declared. A morale check is made by the unit that is being charged – if failed the unit falls back D6″ then regroups (potentially putting them out of range of the charge) and if passed makes the unit count as passing any tests if they lose the upcoming combat.

The book also includes 3 core Zone Mortalis missions, and 2 Apex Missions again to be used as the showcase battle at the end of a campaign. These apex missions are again tied to the narrative of the book and have some fun and unique mechanics.

There’s also a list of unique Cthonia Zone Mortalis terrain features that are used to give the missions some flavour and theme them to the Zone Mortalis battles in the narrative.

New Rules

The final section of the book contains a selection of new rules for Horus Heresy Players.

For Imperial Fists we get a new S5 Breaching 6+ shotgun that can be taken in place of any shotgun in a Fists army.

We also get a couple of new warlord traits for the Imperial Fists, including a really cool one that means instant death only causes D3 wounds as long as the attack came from a Primarch/Titan/etc – allowing your warlord to stand up against the scariest units on the board!

Lord Castellan Garrius is the new Imperial Fists character and he’s an interesting one – he’s been disfavoured by Rogal Dorn and so can never be taken in a detachment with him, but if he fights alongside him (in a doubles game or a game with multiple detachments for example) then he gains fearless and it will not die 5+ in order to try and redeem himself.

His warlord trait kicks in if the enemy is winning, giving him and his unit rage 2 if they ever are losing in victory points. He also has a chance of stopping the enemy from gaining victory points for killing units.

The Sons of Horus get some new Power weapons with AP3 and Breaching that can replace any existing power weapon, and like the Imperial fists they also get a couple of new Warlord Traits

Their new character Vheren Ashuraddon is the First Reaver and has a really cool mechanic that while he has a squad still alive, as long as you allocate wounds to him you can reroll inv saves. Rocking a 2+ save and a 4+ inv this makes him pretty resilient. From a lore point of view he wants the respect of those who follow him, with him taking the bullet for them himself when needed.

He also gets to take a unit of Reavers as a retinue, and gives them line, making him a great option if you are going Reaver heavy.

All legions get access to a new upgrade to Predators, Sicarans and Kratos called a “Decurion Defensor” – costing 20 points this allows you to make an advanced reaction when charged in order to fire all defensive weapons, and fire with pintle mounted weapons with twice the number of shots – A nice way of deterring charges on vehicles.

The second new vehicle upgrade is the “Decurion Locus” – this is 30 points and allows the tank to make the Locus Strike advanced reaction – this is used after resolving a shooting attack against the tank and allows you to either shoot back with all defensive weapons plus one battle weapon, or just a single battle weapon at plus 1 to hit! This is scary to stick on a Kratos and just dare someone to shoot you!

The Sons of Horus get a unique one called a “Lanius” that is essentially a tank mounted commisar – nearby units get leadership 9, but if a unit fails a morale check it instead suffers D3 wounds and counts as passing as the tank shoots at the unit to “convince” it to hang around.

The Imperial Fists also get a unique Decurion called a “Sagittar” that allows a single pintle or defensive weapon to target a different unit to the main target and gain skyfire or precision shots 5+.

We also get rules for the new Cults Abominatio – these are essentially the traitor equivalent to the assassins, but daemon fueled and gribbly! They get access to a really cool advanced reaction with is basically a short ranged teleport shift through the warp, which is as dangerous as it sounds. This can be used in the movement, shooting or charge phase and involves the Abominatio making a morale check, if passed they can move 12″ in any direction (potentially out of range of shooting/charging) but if they fail the roll both the Abominatio and the unit it is reacting to take D6 wounds with no saves or damage rolls of any type allowed as things go horribly wrong…

The first Abominatio introduced is the Infernus Abomination, which is really nasty for 130 points. Rocking Infiltrate, scout and fear and having both a 4+ save and 4+ inv save it should be able to get to where it needs to, and when it does it attacks with 2D6 fleshbane attacks before charging in with a shape shifting weapon – this has a few different forms, but my favorite is the Hammerblade at S8, AP2 with Brutal 2 – with 4 attacks on the Abomination it’s going to sting.

And you’ll want to kill the Abomination in one go, as if for every wound it causes each turn it gets to restore one wound on a roll of 5+

Finally in the book we get the rules for the Inductii – these are force-indoctrinated troops that have been rapidly created in order to bolster the forces due to the massive losses suffered during the years of the Horus Heresy.

These are really fun, and basically for each legion you get some unique advantages, wargear options or bonus at the cost of a disadvantage. So for example, the Dark Angels can take Inductii Tactical Squads – these get to swap their bolters for Volkite Chargers for free, but they no longer get heart of the legion. Or the Blood Angels for example can take Inductii Despoilers who lose site of the legion and cannot make sweeping advances, but the first time they destroy a unit in combat gain fear for the rest of the game.

There’s some really fun stuff here, and it’ll be fun to see how people represent these on the table with different paint schemes and conversions.


I am very impressed with the first book of the Campaigns of the Age of Darkness series, particularly as a fan of narrative games and campaigns this really drives home what the Horus Heresy game is – it’s not really a matched play game, it comes into it’s own when retelling the stories you have read about in Black Library books and giving you a change to command sprawling legions of troops fighting over the battlefields of the 31st millennium.

With half of a 250 page book dedicated to lore, I think that fans of the old Black Books will be happy here from a narrative standpoint, and the missions, campaign and extra rules will allow people to retell these stories on the tabeltop.

The Zone Mortalis rules add something that I personally have been craving for this edition, and this is close confines combat with a lower model count than standard games, and I can see these being really popular considering the lower price of entry with the new plastic Zone Mortalis scenery now available.

Finally we get some fun rules content to add to their armies – and while this is very much not the focus of the book, it does give players some fun new options. Though non-narrative players may not be too happy about picking up a large narrative book just for a few pages of rules. That said, all the non Apex missions also work well as matched play missions, so even if you are not interested in the story there’s a lot of content to sink your teeth into here.

The Horus Heresy has gone from strength to strength and Siege of Cthonia sets a decent benchmark for the future of the series.

Campaigns of the Age of Darkness: The Siege of Cthonia is up for pre order today and is released Saturday 27th May

Games Workshop provided Sprues & Brews with a free copy for review purposes.

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