Perhaps the single greatest strength of the Horus Heresy series is its narrative. While you can point to all the wonderful toys the tabletop game has given marines to play with, and the striking new rules for massed armies, the story was what kicked it off. It got people hooked, invested in the characters and desperate to know more. Even now the novels are still going, building upon the long road to Terra, and supplemented by the Forgeworld rulebooks, So, it was only natural for this brave new universe to be spearheaded by a series of its own, and fill in the gaps left by the bigger books.
Dark Imperium, as a result, sticks close to the core of the main series. It focuses upon Guilliman's war against Chaos, soon after his return to Terra, and his new crusade against the Traitor Legions, specifically Mortarion's sons. However, while Guilliman's new Primaris Astartes wage a war against their older generation veterans, new threats are emerging on other fronts, and Guilliman soon realises that there are more changes to this galaxy than he first realised...
Perhaps the single strongest point of the novel overall is its ability to add more of a human touch which was lacking in the bigger novels. While there were brief moments and odd sections which made for a stronger tale than you would expect in a mass printed book, it often lacked a few of the more personal elements which only novels can pull off. As such, Guilliman himself and his view of the galaxy is vastly expanded upon from the few quotes we got. His dislike for the infinitely less secular M41 is used for equal amounts of humour and pathos at many points, and even his opening scene in the modern Imperium focuses more upon all that has taken place in such a short time. Plus his dislike for the modern Ultramarines who follow Sicarius' example, and the Second Captain's constant glory seeking and title hunting, is a nice clash even if it is subdued.
However, the story also pushes to build links with the older eras and past events to both lessen and enhance the jarring nature of the story's sudden beginning. In terms of lessening it, we see Guilliman's last days leading his chapters in a bid to kill Fulgrim, filling in certain gaps and showing how the Imperium was changing even then; giving readers something familiar to work off of and always hungered to see. In terms of enhancing it, we see Guilliman reacting to the Imperium's semi-feudal nature, noting how far things have fallen since he was leading it. A powerful theme to be sure, and it works well in separating this version of the primarch from his earlier self in the Horus Heresy novels.
The story is also relatively simple and straight forwards, focusing more upon an out and out war with some infiltration themes. However, this works in its favour, as you get plenty of fun explosions, battles and corruption on display, but it works in more than a few new and interesting details. The quieter scenes with Guilliman stand out as he's trying to work with the myriad of armies and regiments of the Imperium add in some interesting notes, while the Primaris marines themselves are introduced with more fine details and qualities than the rulebooks. Their odd link between Guilliman himself and their own primarchs is reflected upon, a number of personal qualities and details are added such as how the Primaris marines were organised and named. Not to mention the effect this has had on the wider Imperium, or how Guilliman is trying to rework its military to more effectively counter the growing threats.
Haley also sticks to his guns with his broad focus, as we get everything from a few auxiliary troopers to a primarch in order to depict the whole war. Anyone who has read Pharos will know how well he can pull this off, and the humans themselves are often pushed about to show more of the overall setting or to offer something for Chaos to work off of. This is certainly a welcome element, but the real strength stems from how they're used to remark upon how un-enhanced humans are used, treated and fight in this setting. Or, at the very least, how they react to the massed presence of Traitor Legions swarming over the entire galaxy, and Nurgle will always seek to use them against the Imperium given their weaknesses to his gifts.
Finally, the battles and engagements are notably very brief but are satisfyingly over-the-top. Apparently to make up for the brief length, Haley just opted to throw in anything he felt would be fun. So, we open up with a full fledged primarch fight, then a massed space battle between two massive fleets, an orbital assault by the new Primaris troops, and the sort of violent meat-grinder which is often only associated with Stalingrand. Also lots of space marines from various primarchs working together.
So, it's entertaining, retains a few great bits of lore to help build upon what came before, and even cuts back to the Scouring. What's wrong with it? A lot sadly.
Now, you won't realise this at first but the story has some major lore issues at various points. They rarely factor directly into the story itself, but they're obvious enough that anyone who has read plenty of 40,000 lore will quickly pick up on them. For example, it's strongly suggested that Guilliman's survival is the result of a mixture of eldar psychic power and Imperial technology, to the point where he is directly told that he should avoid removing it. So, naturally, the first modern scene promptly introduces him without it and even a full armouring up sequence. Minor, yes, but it keeps going from there. Belisarius Cawl is abruptly retconned out of nowhere to be over ten thousand years old, and has been working on the Primaris project ever since the Scouring began, to the point where it's strongly suggested Guilliman himself ordered it. Then, atop of this, you get moments like various Primaris troops somehow instinctively retaining the cultures and attitudes of their chapters despite barley knowing them, and being given to some very odd chapters. The book lists off a few - mostly First and Second Founding forces - but promptly name-drops the Revilers. A group best known for out-Blood Ravening the Blood Ravens to the point where they attacked an Inquisitorial fortress to get at the goods inside and had the Imperium initiate a "shoot on sight order".
Such bits are minor, but that both makes them better and worse. They're never fully in focus so you can skim over them if you focus hard enough just to stick to the main story, but the fact that there is so many leaves you asking as many questions as it answers. Not to mention some very laughable additions at certain points, when it tries to establish a few new ideas, but doesn't fully set them up or properly establish their origins. This stems more from the fact it needs to link into the prior rulebook trilogy and tries to follow directly on from it, but it never explains things. When it does, it's in almost a throw-away manner to the point where the reader is just supposed to accept it, no matter how ground-breaking it might be. Like the eldar siding with the Imperium and abruptly helping to bring Guilliman back to life from a mortal wound.
However, where it seems to suffer the most is in terms of how it simply cannot spread into broader details. Haley tends to work best with a blend of humans and certain emotive elements of the environment. Give him a decaying statue, a pile of corpses or even a whirring mass of alien machinery and he can produce a decent scene. Yet, have him stick characters on the bridge of a warship or in a basic room, and he seems to have trouble adding a little more emotion to the scene itself. He seems to list out the details one after another, and while you can still tell there is a talented writer doing his best with what he has, he's not in his element. This seems to reflect upon the characters as well at times, as the primarchs suffer here. the Fulgrim and Guilliman scene is brilliant to read up to a point, at least until the characters go full ham with their dialogue. This promptly goes to the other extreme with Guilliman's inner thoughts, where more than a few times he's so overly human that it would be hard to think of him as a Space Marine, let alone a demi-god capable of founding a galactic empire. Sometimes it works, but in other times it can really work against the charm of the character himself.
Ultimately this isn't a bad book at all. Haley was given a very difficult task from the get-go, and balancing as many elements as he did was quite admirable, especially when it came to re-introducing Guilliman to the universe. With that said though, the book's structure ultimately works against this. Despite the hard cover and expensive price, this is effectively a light novel, and only the large print allows it to fill out as many pages as it does. Between this and the various very strange inconsistencies, there's a sense that this was possibly rushed out the door to try and capitalize on the new Edition of the game.
Still, for all its flaws, there's no denying it did fill in more than a few gaps in terms of the story and helped to add a bit more flesh to the bones of the new setting. With a look into the new Imperium as it is being re-founded under Guilliman, the Primaris marines and the potential nature of Chaos unchained as it is, there's plenty to like. Pick it up if you're interested in some light reading which explores this new setting, but it's far from being another Horus Rising.
Verdict: 6 out of 10