The Road to Chaos ...

The Soul Drinker Space Marines are loyal to the Emperor who they have served for thousands of years helping to push back the darkness that threatens to engulf the Imperium. While trying to recover an artifact called the Soulspear which is sacred to them, the Soul Drinkers watch it get stolen from under their noses by the Mechanicum who want it so they can study and reverse engineer it. The Soul Drinkers obsession drives them to attack their allies in an effort to retrieve the Soulspear, but they cannot and are forced to flee into the warp.

The Soul Drinkers soon find themselves declared Excommunicate Traitors and all the forces of the Imperium are turned against them. Meanwhile inside the Chapter the Space Marines themselves are changing and showing signs of mutation, and have begun to worship the Architect who they see as the Emperor without the corruption of the Imperium. But things are not as they seem and it is revealed that the God they are worshiping is very different to the one they believe they are following.

Ben Counter's series about these Space Marines who aren't part of the Imperium but refuse to follow the Chaos Gods begins in Soul Drinker and continues with the Chapter trying to regain their lost honour and be welcomed back into the Imperium. I'm interested to see where Counter goes with this series in the future.