Title and subtitle are as far as the prominent two plot threads go. Besides some embellishment in form of some (to fans of the W40K universe) known rivalry between Legions (eg Space Wolves/ Thousand Sons) you get exactly what you'd expect. Embedded in a mix of martial action, gore and cosmic horror, is a fine, though somewhat superficial discourse about the meaning of brotherhood. Fans of space or rather warp travel get a believable description of the Immaterium, based on colours and emotions. All that written in diverse language, depending on which Legion's character is the point of view.