How do you review a Black Library book? For one, you probably don't rate it. The accomplishment of this text as a novel isn't so much the point: it's to add to the flavor & worldbuilding of the lore of Warhammer 40,000, for those interested in that for its own sake as well as those playing with the miniatures and open to inspiration.
This text, even in that regard, still isn't all that great. There are some emotionally powerful moments when characters are entering battle for the first time or realizing their potential. But the prose suffers from too much repetition of its grimdark phrases. And the perspective shifts feel more about an author needing to fill pages than to the service of the narrative. Plus, the final battle is as over-extended as anime filler.
To that last point, this novel is in the action genre. For those keen on descriptions of battles, there is plenty to offer here. But for someone like me who gets the gist after one such longform description and is more interested in voice/style/consciousness and small-scale yet profound situations in my fiction, this is merely audio to paint miniatures to.