I found this an astonishingly compelling book. I greatly enjoyed the previous books in the series, I thought ADB had made a grand effort to put his own spin on a Legion already covered brilliantly in Simon Spurrier's Lord of the Night. But with Void Stalker, ADB has taken everything that worked well in Soul Hunter and Blood Reaver, turned the dial up to eleven, and added some fresh ingredients into the mix for good measure (if you'll pardon the tortured metaphor).
First, Talos himself. This is a triumphant portrayal of a superhuman suffering suppressed and all-too-human doubts. The gradual realisation that his body is failing him - but that with his death will come the opportunity of new life, which may succeed where he was doomed to fail - but that he realises that he doesn't even know what success means, or what it will look like when it happens, or how to ensure that the prophet who follows him will measure success in the same way (given that so many in his own Legion, company and squad disagree so fundamentally); this is a sensational character, rendered magnificently. I have never felt so desperate for a character in a BL book to thrive.
Next, First Claw. Despite having already read of their exploits for two novels, an audio drama and a short story, I sometimes felt that they were a bit... interchangeable. Uzas stood out for obvious reasons, but the rest were sometimes little more than mirrors in which to reflect Talos. That was not the case in Void Stalker. I understood them in this book, and related to them in an entirely unprecedented way. They don't like each other - they are not friends, but they are brothers. They fight together because, well, what other choice is there? But then what are they fighting for?
This very question is one that ADB explores in this novel and the partial answers that he finds forced me to demand of myself a reassessment of the entire concept of Chaos Space Marines, or even of the entirety of non-Imperial humanity in the 41st millennium. They are fighting a war they lost ten thousand years ago (even if they've "only" experienced a century or so of the intervening time themselves, thanks to the vagaries of the warp), but to an ill-defined end. Even Uzas (with whom I sympathised greatly while reading this story) isn't entirely mindless in his worship of the Blood God; Xarl's death brought tears to my eyes; Cyrion and Mercutian turn the concepts of loyalty and treachery, nobility and cravenness on their heads. Then there is Variel, whose motives align entirely with none of the others, but who may perhaps be the purest if viewed in a sufficiently stark light. Every last one of them felt real to me, in a way that "evil" fictional characters rarely do. They represent the power of Chaos, but not as skulls-and-spikes maniacal laughter pantomime villains; they do some truly terrible things but I still found myself empathising with them; ADB's Night Lords are grim, and they are dark, but entirely avoid the one-dimensional stereotypes of "grimdark" 40K.
So ADB writes Astartes well, I'd be surprised if anyone much disputes that; but the whole series has followed Octavia's experiences, and she and Septimus are the perfect foil. Human (more or less) where the Night Lords are demigods, resigned to their fate but with their ambitions for freedom rekindled when they undergo a shared experience which the Astartes cannot share or even begin to understand; yet all the while retaining a loyalty and devotion to their dictators. (Septimus telling Talos to "eat ****", having most of his bones broken, and still acting how he does for the remainder of the book, was simply a wonderful sequence of scenes that I didn't want to stop reading). I can't emphasise enough how powerful I found this.
If I were held at gunpoint and forced to come up with something to criticise about this book... some of the Eldar sequences didn't fully mesh for me. The poetry of the Legion returning to Tsagualsa (even the name sends a shiver down my spine, now) and burrowing into the catacombs to make their last stand felt a little underwhelming. In part, this may have been intentional; the Eldar ships were largely operated by spirits, and it was deliberate that their ground forces were not as overwhelming as their fleet had been. It still felt a tiny little bit anticlimactic, though, and the fact remains that Jain Zar, for me, just didn't really work.
That's a minor criticism though, because fundamentally this book isn't about the Eldar. It's about mortals and demi-gods, and how easily the outrage of the latter can devolve into insanity when they consider themselves to have been wronged. Konrad Curze sought vindication in death; but his sons are not as he is, and although they seek vindication and in a sense they seek death also, they each attempt to resolve these tensions in their own way in a galaxy that has damned them. The result is a gripping narrative magnum opus that anyone and everyone who has even a passing interest in 40K should read.
tl;dr: this book is amazing.
Finally, if GW don't make Decimus a playable special character in the next CSM Codex, I will march on Lenton with a flaming torch and a pitchfork.