There's a lot going on in this book - in terms of lore, in terms of plot advancement for 40k, and perhaps answering a few questions about one of my favourite legions from 30k: the Alphas.
Firstly, I must admit that I really liked the characterisation of the 40k Alpha Legion. As with my own prejudices, these Alphas have not fallen to, or committed themselves, to Chaos. In many ways, they are "pure" - or at least free from corruption to begin with. In some ways, they reminded me of the Soul Drinkers series of books - a chapter that was freed from the Imperium, but still doing the "right thing" ... sort of. Except this time, they were totally working for themselves.
Finding themselves marooned on a world where they'd killed most others, the Unsung (as this Alpha Legion Harrow calls itself) finds a way off by agreeing to undertake a mission for the Emperor's Children (who seem to continuously think they're the best, but seem to get outwitted mostly ... except for a potential twist at the end).
The story focuses on the Alphas taking out a plot maguffin (I don't want to spoil it) for the glory of the Emperor's Children. To do so, they need to enter a war zone and get there before a large contingent of World Eaters get to the self-same plot maguffin. Opposing them are the Imperium -- they're fighting on an Imperial world behind the new Warp incursion in 40k which is a lynchpin for the local segmentum.
Along the way, the Alphas fight to not become corrupted -- some of their number really want to accept the "rewards" of chaos they have felt they have earned; there's a daemon sword that's also trying to corrupt them. Most are strong enough to resist, others are not. Some want a different way to fight. Others want to just go their own way.
We see the Alphas fight, but not as they did in 30k. They have a lack of resources, and are forced to have to be very inventive along the way. I really liked this evolution and characterisation of desperation. I genuinely hope that they will release another book on the Unsung as I'd certainly buy it. Its one of the better publications recently in my mind.
The negatives were probably the appearance of a certain Imperial Saint. I get they she might be there, but its just a little bit on the implausible side for me. The appearance of Kharn the Betrayer was also a surprise. I don't know why the World Eaters with some semblance of sanity still tolerate him being around. No wonder he almost fights on his own (with daemons). That said, all blood is welcome I guess. I had less problems with Kharn than I did with the Saint.
Other than that, a great read overall!