I'm a fan of the Horus Heresy series and a big fan of Dembski-Bowden who is one of the outstanding writers in the Black Library stable. His writing here is of his usual high calibre with punchy, immersive battle scenes and some excellent characterisation but the problems with the novel are too big to ignore. Some minor spoilers below -
One of the highlights is his sympathetic portrayal of rebellious primarchs Angron and Lorgar. They are shown to be warm, thoughtful, intelligent beings, neither of whom believes they are evil but rather are victims of desecration of their honour by the emperor and the Butcher's nails (Angron) or doing what is necessary to lay bare the emperor's lies and reveal an, albeit unpalatable, truth (Lorgar). Their monstrousness is more subtly revealed by the barely mentioned but obvious fact that the background to their actions is the destruction of civilizations and the ending of billions of ordinary human lives, which they seem barely to notice in pursuit of their personal desires. The background to why Lorgar and Angron are as they are is explored and draws the chain of events leading to the heresy past Horus' vanity and Erebus' machinations back to the Emperor's own actions more potently in this book than in most of the others in the series.
Battle highlights are the titan battles and void combat. There are also great non-combat plotlines such as the continuation of Argal Tal's storyline in an unexpected but satisfying direction and the development of Kharn. There are great new characters such as Lotara Sarrin the world eaters flag-captain and other human and mechanicum characters. In the great non-astartes characters he creates the author tackles the question of how the World Eater's legion could be kept running if all Angron and his space marines want to do is charge the enemy head on at every opportunity. Its a well done reversal of the usual 'even though they were created to be warrior-servants of the teeming human multitudes the god like space marines do everything brilliantly and just allow the humans to tag along'. How Angron's insistance on his marines having the Butcher's nails is destroying the legion and how they cling to brotherhood as everything else that usually defines a space marine is stripped away is movingly explored.
Unfortunately there are also such big holes in the novel's plausibility that they can't be covered over by the quality of the description and characters.
Running at the enemy whilst shouting wins everytime:
The problem is that in highlighting the problem with the Butcher's nails and the importance of the non-marine characters, Dembski-Bowden repeatedly points out that the World Eaters are like rabid animals with poor tactics, poor unit cohesion, poor communications, little battlefield command (what tactical direction there is comes from the human flag-captain), friendly fire incidents, little use of combined arms (eg their titan legion bemoans its loss rate as higher than other legions because the world eater marines just don't work in concert with them), poor battlefield discipline, a high casualty rate etc. Despite this we have to believe they slaughter their way through vast numbers of Ultramarines, the most tactically sophisticated, numerous, disciplined, brilliantly led etc legion, on their own territory. And the reason they can do this? Well, its repeatedly explained that its because they are aggressive. Snarling and waving your chainaxe around whilst charging at the enemy slightly more often than you charge your own battle brothers pretty much trumps any fancy tactical, superior firepower, or other nonsense the enemy might try and will always win the day. And even if your casualty rate is stupidly high your legion will somehow never get worn down by attrition. Its so daft that it seriously undermines the whole book.
Nobody minds being a traitor:
The World Eater human and mechanicum characters in the story are all well described and easy to relate to. They are described as normal people doing their various military jobs. In fact, if you weren't told they were working with the World Eaters you'd probably assume they were loyal imperial citizens. There is absolutely nothing to indicate why any of these characters has turned against the Emperor. One of the major mechanicum characters even keeps a scroll message sent to him from the Emperor because it is precious to him and seems to still consider the Emperor as the Omnissiah he worships.
Even odder than this is the reaction of a squad of World Eater dreadnoughts who are woken up having, with one exception, been asleep since not just before the heresy but before Angron was even found. The exception is the former legion master, appointed by the Emperor himself to run the legion until the primarch was found and who ended up in a dreadnought sarcophagus because his mad primarch nearly killed him just after his discovery. On being woken they are given a data upload which explains that their new Primarch - whom some had never met -is part of a rebellion to overthrow the Emperor - whom they were all loyally serving when they went to sleep - and they now need to go kill some loyalist space marines. They all just go off and do it without any indication of being troubled. 'Hey wake up. Listen, I know you loved and served the Emperor and were willing to give your lives for his vision of a galaxy wide imperium where humans would be safe and prosperous when you were last awake but we want to destroy the imperium you nearly died to forge, slaughter billions of the innocent then kill him and anyone who serves him, is that OK?' 'Uh, yeah, sure, count me in.'
Legion fighting legion? That's unthinkable before the heresy..oh, no, wait, no it isn't:
One of the tropes that has kept recurring throughout the Horus Heresy series, especially in the early books, is how unthinkable marine fighting marine was before the heresy. Most major characters have agonized over it at some point, the news of it happening has been greeted with shock and disbelief, an ultramine character in Know no Fear was punished for having even contemplated the possibility of it. In Betrayer, the author casually drops in that the Space Wolves and the World Eaters had a full on pitched battle well before the heresy, used by Leman Russ to try to teach Angron a lesson. Which rather makes a mockery of the 'battle brother against battle brother has turned our whole perception of reality upside down its so impossible to even comprehend' lament that characters in the earlier books keep spouting.
Finally there is a personal annoyance - In the 40k universe humanity has spread across the galaxy to every imaginable ecosphere over tens of millenia but, with the exception of the Salamanders chapter who are all black, everyone seems to be white (or sometimes 'dusky' (ie meditteranean)). Not only in the novels but the artwork on model box covers, books, posters or the painted models in White Dwarf every month. Dembski-Bowden tackles this by pointing out in his novel that, unlike other chapters, the World Eaters are drawn from a vast mixture of ethnic types. He points this out immediately before having the only obviously black character, Delvarus, (who comes from a jungle, of course), being taught a much needed lesson in honour and brotherhood by a number of his more noble comrades who, where their ethnicity is described, are white. As one scene in one novel it doesn't really matter but, rather depressingly, it just seems part of the same unspoken and pervasive leaning in GW products.
So, the great writing we have come to expect from one of Black library's best writers, marred most particularly by the impossible need to make the utterly tactically inept World Eaters conquer everything in their path and for important characters to be traitors without any obvious reason. Buy it second hand on ebay, don't spend a tenner on it new.