Spear Of The Emperor by Aaron Dembski-Bowden.
The story takes place post Imperial Nihilus, about a 100 years later. Amadeus Kaias Incarius of the Mentor Legion is sent through the Great Rift to the scattered worlds of the Elara's Veil nebula. This area of space is oath-sworn protected by the Star Scorpions, Celestial Lions and the Emperor's Spears. With the Scorpions basically destroyed, and the Celestial Lions still recovering from Armageddon 3, the Emperor's Spears are shouldering the brunt of the work protecting Elara's Veil.
But here's the twist, at least from a narrative standpoint: the story is a first person perspective accounting told by one of Amadeus' serfs. Helot Secundus Anuradha. She is the second ranking amongst the trio of attendants serving Amadeus. Helot Primus Kartash and Helot Tertius Tyberia are also in attendance.
Upon arrival to the chapter home of the Emperor's Spears, Nemeton, the reception is cold to say the least. The Spears are a gruff sort of Chapter and their culture seems primitive to outsiders. They are loyal but not quick to trust outsiders. But Amadeus, with the assistance of Anuradha, eventually win over the Spears through the revelation of secrets, the execution of prophecies, and as brothers in arms in war against the Exilarchy, a Chaos force lead by the Renegade Space Marines of The Pure.
The helots of Amadeus and Amadeus himself are captured by The Pure halfway through the book and the tone of this gets dark. Darker than grim dark. And just when you think you understand who the enemy is, who the antagonist is that you need to keep your eye on is, you find out Amadeus' whole mission was veiled in a lie. By the end of the tale Amadeus has been rebuilt in all aspects, from the inside and the outside, as he sheds the lies of his past.
I want to tell you more, even put up a spoiler warning and let my thoughts dwell on some of the juicier aspects of this tale, but I won't. And it's for the simple reason that I recommend this book so highly that I want you to read it. My recanting, even in the most vague of summaries, would be a grave injustice to this tale. Find this book, in whatever format, and read it.
This book is simply phenomenal. Is it a must read? No. But if you read it you will not be disappointed. This might be the best Space Marine centric story I have (so far) ever read. If you see how many reviews I have posted you'll know I have read a lot of "bolter-porn" over the years, so I have a lot a material to gauge a story like this against.
If you want to know more, I highly recommend Arbitor Ian and Mira's book club review. Or you could just download the book and dive right into the worlds of Elara's Veil.