I did things slightly backwards, but that's okay. I read The Gate of Bones first, which is the second book in the Dawn of Fire series. So reading Avenging Son ended up as a prequel to me, even though it's the first book of the series. Star Wars prepared me well for such things.
Anyway, Avenging Son - the primarch reborn, Roboute Guilliman, launches the Indomitus Crusade, a military undertaking that eclipses all others in known history. As vessels in their thousands burn through the cold void, the attention of Fleetmistress VanLeskus turns to the Machorta Sound, a region under attack by a dreaded Slaughter Host of the Dark Gods. The success of the Indomitus Crusade will be determined by this conflict, and the desperate mission of Battlegroup Saint Aster, led by Space Marine Lieutenant Messinius.
The story is told from a few perspectives. Messinius and the new Primaris Marines give good contrasting accounts - Messinius is confident in not just his ability but in himself until he realises that the Primaris are really his replacement and other than experience, they're better than he is. The Primaris Marines know they're good but filled with so much self-doubt about how they fit in and their experiences. I particularly liked how they addressed that the Primaris are stronger, faster and all-around better equipped than a normal Space Marine, all except for those decades of inexperience that means they lose some of that edge. They question things, they worry about how they will do considering that they've never actually been to battle or killed anyone/anything before. Showing the inside of their minds before being awakened was something else and absolutely brutal - they die in the simulation and are told that the simulation will restart and they need to do better, kill more before they are killed. Basically told in no uncertain terms that they are a weapon and expected to die, but their aim is to take down the maximum number of whatever enemy they are against with them.
There are also some human elements mixed in with the Space Marines which add a different dimension to the Space Marines. These aren't people that are weapons, they're just people. Some with a higher purpose than others, like Inquisitor Rostov of the Ordo Xenos and Imperial Guard Lieutenant Lacrante, whose story I wanted more of. The team that Rostov is building and showing it from Lacrante's view could be in a book of their own and it'd be brilliant.
Then there's a small subplot about a scribe going through an Emperor inspired adventure through the hives of Terra, to bring a missive containing a warning about the Pariah Nexus. This sadly really became a subplot as it had a few chapters and then vanished for a massive part of the book. This story though is dark and depressing, which is well suited for a Warhammer 40k book.
Other interesting parts include showing how there's clear distrust between Guilliman and Cawl. Cawl is a showman, making announcements and displays to the Primarch with maximum flamboyance, and his comments on the history of the Space Marines was a real highlight, and it makes sense that he would know more about how they came to be and why than anyone else alive.
AVENGING SON takes a while to really get going as you jump from character to character, but when it does you're in for a ride, particularly as certain characters stories intertwine and the crusade kicks into motion. It's well-written, packed full of interesting characters and stories, including one of the more depressing W40K subplots than I can remember. Life on Terra is bleak!