For the last three books (ignoring the anthologies, of course) the Heresy has been closing off plot lines. The fate of several legions has been dealt with, a few long-standing questions resolved and side stories closed off. We've had the end of the Imperium Secundus, seen what becomes of the Space Wolves and both the White Scars and Blood Angels are at Terra. So, Slaves to Darkness does the next thing the story needed: Unity. While the subtitle might be "Chaos undivided" a more accurate one would be "Let's get the band back together".
Horus has fallen. At the moment of his legion's triumph in breaking the Imperial blockade to Terra, the wound inflicted by Leman Russ has torn open once more. It is more than merely a mortal blow, and his very spirit has seemingly been splintered by it. As the Sons of Horus fight to disguise this truth from his brothers and hold the battle line together, other primarchs are dispatched to find their wayward brothers. With the Siege of Terra upon them, it is time to gather every remaining astartes within the traitor legions and march on the Emperor's palace.
Surprisingly, one of the big highlights of the book proves to be Perturabo. John French's depiction of the character has not been a popular one in the past, often reducing the Lord of Iron to a screaming maniac. Along with the Forge World rulebooks, it seems to have approached him with the impression that he's more interested in finding excuses to kill his own troops than effectively leading his forces into battle. This was most obvious when compared with the (if somewhat problematic) more detailed look at the character Angel Exterminatus offered, which gave him some much-needed depth. More than a few people might have expected the return of the hammer-wielding madman, but the Perturabo we get here is far more tempered in his nature.
The Iron Warriors primarch is blunt, expects total obedience and will bump off officers at a rate a Commissar would baulk at, but it's not without reason. The way he's written doesn't make it seem as if he's looking for an excuse to kill everyone around him, or simply has rage as his only emotion. It's far closer in nature to the original Index Astartes source material than with many past works, and what we have here more than makes up for a few past mistakes.
Equally, Fulgrim has undergone a smooth transition to his daemonic self, shedding the last few humane qualities which clung to the character. He's undeniably Slaaneshi, and the way in which he revels in his excesses is certainly something we have seen before. However, the use of call-backs to his past self and grim reflections of prior books. While this is true of Angron as well, Fulgrim's nature allows him to converse, explore and respond in more ways than simple violence. Combined with the openly flippant and unconcerned nature, it makes him an amusing contrast to the more dedicated primarchs. As a quick example, when he's found in this book and asked to take control of his legion again, he's living it up on a daemon world created by his patron god, and openly tells his brothers to bugger off.
The reason I highlight these two, in particular, is that the book needed a solid basis for the others to work from and build the rest of the narrative around. Without that, the story would have been utterly overburdened in trying to divide its focus between so many different primarchs, their subordinates, and other characters besides. While the likes of Lorgar and Malgohurst also serve as a means to drive the narrative forward - and it's always nice to see the Twisted take a front row seat again - it needed a bolder and more brazenly examine the inherent problem the traitors suffer: Chaos is chaos.
That comment might sound like an obvious one, but all too often Chaos itself is treated as an ordered and regimented thing. It's more a way to show someone go a bit mad, glue some spikes onto their armour and then fly about with all these new daemon powers from one of the four gods. Despite all the various novels which directly contradict this, the general fandom view of this does tend to categorize and label Chaos in this easily defined manner. It's something which is admittedly not helped by the use of daemons emulating their tabletop models, but that's an unfortunately unavoidable issue when it comes to this sort of thing. Chaos, as it's shown in this book, is self-destructive, completely corroding and far from this path to easy power than you might expect. many points serve as a slap in the face if you're wholly aware of its self-destructive properties, as it finds ways to constantly remind you of just how this effects anything on a large scale.
The traitor legions as they are will not exactly be bringing their A-game to Terra. Many have lost more than they have inherently gained with their alignment to Chaos, as they have suffered a severe breakdown in discipline, supplies and control since the start. While The Path of Heaven had briefly cited this with Horus' conversation to Mortarion, Slaves to Darkness truly shows it. It explores it, it details it, it takes a massive great spotlight and shines it on the legions saying that "This is Chaos", with the larger forces working almost in spite of themselves. Because of their disorganised nature and lack of true investment in the wider war, Horus' strike on Terra has become as much a decapitation effort as a hail mary pass. His legion's way was to claim the heads of the enemy leadership before dividing and destroying the body. Yet, this has turned into an effort to execute the strike while he still has forces who will effectively coordinate such an attack.
What should be praised with French's efforts is that, while this is an exploration of Chaos' weaknesses, it never tries to make the legions themselves truly weak. It avoids the Iron Hands syndrome of taking the message of weakness and emphasising it or exaggerating it until any strength is wiped away. For example, it's made clear that Angron is a loose cannon and a monster who will butcher everything in his path without pause or remorse. Yet, even as it details this, it never downplays the fact he can murder everything in his path, and Khorne's blessing will allow him to solo whole armies at a time. The powers of Chaos can teleport entire legions across light-years of space, bend the fabric of reality and bring a man back from the brink of death. So, while it might show how the empire Horus dreamed of was destined to fail, it never downplays the individual benefits of the Ruinous Powers.
The last point of praise - the last one I can praise without spoiling some of the best bits of the book, such as the saga of a certain Iron Warrior - is its use of scale. We all know that the Siege of Terra is going to be huge. Really, it's the big battle of the setting, with a vast engagement so huge that it is a war unto itself. As such, a few writers might have made the mistake of trying to directly compete with the Siege on that front, but instead, it opts to use scale in a very different manner. By having the viewpoint characters be so diversely scattered throughout the galaxy, by having each repeatedly call-back to past events and story arcs, there's a true sense of immensity to it. It feels as if this is building toward a storm, and serves as a reminder of just how huge all that has come before it truly was.
Even when the book does delve into bolter porn, it's well-timed and extremely well planned. Much of this surrounds the Iron Warriors, but it's used to comment on the state they are in. It draws attention to how the world has changed and ultimately what has become of the well-supplied supplied forces which once made up the legions. This is most evident during the rearguard actions against the Ultramarines and their allied battlegroups, but it even shows up on a very ground level view. Away from the primarchs, the gods and the prophecies, you can see how this has reshaped the soldiers fighting in them even when they are just astartes fighting other astartes bereft of Chaos' direct influence. Plus it even tries to deal with one long-standing issue of casualties, but that does, unfortunately, open up one possible plot hole as it is.
So, with that final note, it's onto the bad parts, as you might imagine.
The book doesn't know how to use all of the primarchs. That's all that needs to be said at the start here: It doesn't know how to fully explore and examine each in the right way. This results in several having little more than cameo appearances in the final chapters, mentions or even existing in the background. This could easily be forgiven to a point, but even those which are given the spotlight fail to fully stand out. The reason Perturabo and Fulgrim's roles in the book were so openly praised is that without them this entire novel would have failed. While Lorgar does play an essential role within the story, his presence seems to be there as a mere vehicle. He exists to show off a few interesting scenes while punting the story along to a new location, and his own personal developments only emerge very late into the story. Angron suffers from a very similar issue, in that he exists largely as an obstacle for Perturabo rather than offering more insight into his new daemonic state.
While you might have thought that Horus would be at the forefront of this, even that isn't true. The few moments he gets to highlight his character almost purely emphasise the past and serve either as flashbacks or minor conversations. He's sidelined throughout much of the tale due to the after-effects of his duel in Wolfsbane, and this only hurts the book. Despite the fact his very name is in the series headline, we have seen very little of him overall in this saga. The last time he took any role of true prominence was all the way back in Vengeful Spirit (a deeply flawed book to say the least) and given how his final moment will soon be upon him, he needed more time devoted to his character. By sidelining and limiting his presence here, it didn't hurt the book but it seems like a move which will hurt the series.
What was definitely a much more negative move on the part of John French was how important the Sons of Horus truly are within the story. Compare the opening trilogy with this book and you'll note a number of major differences, the least of all is how it fails to give Aximand and Abaddon anything of relevance to do. While the Mournival is supposed to be a reflection of Horus himself, and a vital part of the legion, it's all but forgotten here. Remember how Aximand was traumatised by his need to kill his brothers? Forgotten and discarded. Remember how Abaddon effectively ceases to exist for whole eras of the series? That comes back in full force here. Multiple C-list characters take their place, and this only further undermines the legion as a whole as it robs them of an opportunity to explore their identity.
Now, as great as the initial trilogy was there's no denying that the Luna Wolves lacked something in comparison to the other legions. They were not nearly so solidly defined in terms of internal culture, style and visual characteristics. That was because this was Horus' story at the time, and that of Loken. Yet as time moved on, more and more novels began to better utilise the internal cultures of their legions, from the Thousand Sons to the World Eaters. Slaves to Darkness could have corrected this - it certainly had space, but it, unfortunately, failed to use it effectively. Instead, it uses the time to try and flesh out characters who will never be seen again beyond this book, or restore a status quo in time for the Siege itself. This is to say nothing of a major twist involving Maloghurst which was likely intended to be tragic and impactful, but it comes across as infuriating due to its timing and delivery.
The final issue is perhaps the greatest problem which has plagued the Horus Heresy series since the day Fulgrim was published. While some books veered away from this and some actively tried to correct it, time and time again the stories of characters would push too far forward. The state of the galaxy, the state of the legions, the situation with the primarchs themselves, everything doesn't seem like it's from M31 anymore. It's all too close to M41, and in the space of fifteen years the galaxy has more or less reached the state it's supposed to reach in ten thousand. While the Horus Heresy is definitely an integral part of the setting - arguably the most important chapter of its history - it should have been the start of the decay which set into the Imperium. Instead, it's already reached a point where we're now supposed to believe it will remain at, without any change, for the next ten millennia.
At the end of the day, Slaves to Darkness is still a John French book. It benefits from his descriptions, still and punchy if poetic descriptions, but it stumbles at a few too many points to truly stand out. As result, it's a serviceable tale with some decent moments and interesting concepts, but that's it. Those fully invested in the series will want to get this one due to how it sets up the final arc of this long-running story, and for its more engaging chapters. At the same time though, you will need to stomach as many disappointing moments as great ones to get through this novel.
Verdict: 6 out of 10