From the cover and fluff, you might think this is just a shoot'em up story about the space marines, the 40K Universe's ultimate human warriors. This is deceptive and I am glad that it is! There is a lot more to the book than advertised.
This includes excellent characterization of characters, including humans, aliens, and secondary roles. The plot was also very well drawn out. The horror of the times and various aliens and sociopathic humans is well presented, gruesome but not overdone and ridiculous. If I had to summarize it, the book is a combination of Stephen King horror, gritty sci-fi, and war novel.
It is also faithful to the fluff of the Games Workshop 40K world. If you are a gamer or even just a reader of that imagiverse (which is beginning to rival Tolkein's Middle Earth in breadth and depth...granted, only through the efforts of hundreds of people!), this book fleshes out many plot details and events of 40K.
Basically, the plot revolves primarily around a missing Necron vessel that has been haunting the warp for millions of years. A complicated set of schemes involving the dark eldar, the humans, psycho evil humans, and the faithful Space Marines emerges with various characters trying to stop others from locating the missing ship and the C'Tan god known as the Nightbringer (which is why the title of the book suits it. So it is NOT just a space marine novel, it is really much more.
I would recommend this for anyone who was interested in gritty, action oriented sci-fi with the usual plot twists and teases to keep you excited and reading. You should also be able to tolerate explicit violence, including torture, which crops up occasionally. If you like that style of book, you will love this book. If you want Star Trek style heavily cleansed sci-fi universe heavily guided by modern morality and delicate sensibilities, this probably isn't for you.