Gentle reader, are you familiar with the writing convention wherein the author will build up tension in a series of long, descriptive sentences? Where details and exposition are layered on, setting up a scene in which it is clear something pivotal is about to happen? The words pile up and the reader is primed for a big reveal. Then, the author will break from the previous loquaciousness of prose displayed thus far and punnctuate the action as with the following device.
A single short sentence.
When used sparingly, this can be very effective in entertaining the reader. When this technique is employed so often that it feels like it's appearing every other page, it wears a bit thin.
It doesn't help that this is a completely superfluous story that hardly merits inclusion in the Horus Heresy. It could have been a passable short story, but as a novel, this work flounders.