The strange thing about Henry Zou's 'Emperor's Mercy' is that it is an absolutely awful book, but might have been two or three fairly decent ones.
Let's just get this out of the way; Black Library books are hardly high art. Some of them are better than others, but the best are usually just really well-written tales of martial prowess and total war, with the odd bit of philosophizing or thriller-mystery action thrown in for good measure. When done well, they're fun, pulpy reads. When done poorly, they're a slog of badly connected fight scenes and utterly flat characters. And unfortunately, despite what Dan Abnett's quote would have you believe, 'Emperor's Mercy' does it very badly, indeed.
Emperor's Mercy is the story of Inquisitor Obadiah Roth, sent on a mission by a senior inquisitor to discover the identity of the fabled 'Old Kings of Medina'. Sent into the Medina Cluster, a multi-planet system, in the midst of a major Chaos invasion, Roth's mission is understandably complicated. Travelling to three worlds in the system, picking up and losing companions, and of course getting into all manner of scrapes along the way, Roth ultimately learns the truth about the 'Old Kings', the purpose of the Chaos invasion, and the ultimate cost of protecting the wider Imperium from both.
It all sounds solid, but unfortunately it's executed absolutely execrably. The problem is, Roth's adventures feel less like a constant narrative and more like a series of barely-connected incidents. There's no real sense of scale; the planets he visits feel like they're no bigger than those the Enterprise or SG-1 might go to, consisting of one city and perhaps a dozen characters who actually matter, if they're lucky. And there's no particular sense of either time, either, with whole weeks of running and fighting on Cantica glossed over in a sentence or two, while three days on Aridun take up chapter after chapter. The story could take month, or a year. And so much of it is just wasted, or has no real pay-off. What little actually matters from Cantica could've been folded into either of the other two planet's sections with no loss, and considerable improvement, to the overall narrative structure, and frankly Kholpesh and Aridun could've been collapsed into one planet, too. The book is terribly scattershot, which makes it hard to really get a sense of how things are going, and why. This isn't helped by the fact that major military forces and characters, like Inquisitor Gurion, the 9th Route Fleet, allied Space Marines, and at least one, and possibly two, companies of Blood Gorgons just disappear from the narrative for no adequately explained reason. While the individual action scenes, taken on their own, can often be rather well written, overall the story of the war between the Imperial Guard and the forces of Chaos in the Medina Corridor is just a complete mess.
The worst of this book's failings, though, are in its characters. Where an older, more experienced, more established Black Library writer might spend book after book on a single Inquisitor and their retinue, Zou introduces no less than five, none of whom really emerge as fully realized characters. Delahunt, Barq and are Celeminé are ultimately ciphers, to varying degrees, and Gurion, as mentioned earlier, just disappears from the narrative after a relatively bad-ass moment two-thirds of the way through. That just leaves Roth himself, and he isn't much better. He is sometimes stoic, sometimes suicidal, sometimes courtly, sometimes abrasive, sometimes cold, sometimes emotional, sometimes suspicious, sometimes naive, sometimes young and inexperienced, sometimes well-read and intellectual (apparently)... And while such contradictions can be used to construct a whole greater than the sum of its parts, here they're just kind of a mutually contradictory mess. And the supporting characters are pure stock; Lord Marshal Khmer is 'the needlessly obstructive rival', Silverstein is 'the faithful friend/the sniper', Captain Pradal is 'the young but determined military officer' and Madeleine de Medici is 'the lady intellectual/the archeologist'. None of them manage to grow beyond those easy labels, and you know about as much about them at the end of the book as you did at the beginning. Compared to things like The Fall of Prospero, Firewarrior, the Eisenhorn trilogy or the Ciaphas Cain series, this is pure amateur hour. Heck, as much as I absolutely loathed the way Alpharius/Omegon came to their decision at the end, even Legion did a better job of building characters than this.
There's one last thing I want to say about Emperor's Mercy, and it's a bit spoilerish, so be warned. There's a sub-plot running through it about a traitor within Roth's group, which presumably is how certain groups are able to find and attack them. This is dumb enough, since of the two attacks one would have killed the traitor, as well as the target. But when it's finally revealed who the traitor is, it goes from being just kind of dumb to outright idiotic. There's no real justification for it, either in the narrative or on the part of the character themselves, with the traitor actually going so far as to just cut off Roth's demand for an explanation and start a fight. To me, that's a bald admission that Zou didn't really have any idea about why this plot-line was there, other than possibly because someone told him it should be. And what little justification that's there actually makes things make even less sense; the traitor claims that working for Khmer against Gurion's mission will lead to them being elevated in rank, but really? The favour of a Lord Marshall means more than the disfavour, and probable extreme suspicion, of a high-ranking and highly-respected inquisitor? That was, by far, the dumbest part of this book, and this was a book that did not particularly lack for bad plotting and nonsensical motivation.
Also, at one point Zou refers to a gathering of some Imperial Navy, Guard, Astartes and Assasinorum leaders as having enough combined firepower to kill whole galaxies. Really, man? Whole galaxies? Would that be why they're having so much trouble defending a single solar system against some raiders and a company of Chaos Marines, Zou?