The Lion (The Horus Heresy #Novella)

With their crusade against the Night Lords reaching its height, the Dark Angels find themselves thrust into a new battle. Beset by daemons and forced to choose between two brother Legions, the Lion's loyalties will be tested and a secret will be discovered that could change the face of the war, and the destiny of the Dark Angels Legion.
Read it because
The Lion is finally forced to choose as a side as the Iron Hands and Death Guard come to blows over a mysterious artefact. And when he claims it for his own, the Dark Angels primarch sets his Legion on a course that will haunt them for ten thousand years.