None More Loyal (Black Library Advent Calendar 2014 #14)

Crimson Fists Sergeant Galleas and his squad are assigned to aid an inquisitor who hunts a notorious renegade Space Marine. Setting out to entrap the traitor, the Crimson Fists soon find themselves surrounded and in danger - and their enemy may know them as well as they know themselves, as an ancient and deadly secret is revealed. Read it because
The Crimson Fists have not had the best time of late - what with their home world near destroyed by an ork Waaagh! under mysterious circumstances. In this new story, their loyalty to the Imperium is pushed to breaking point as they come into contact with shadowy agents of the Inquisition...