Daemonology (The Horus Heresy Short Story)

Chagrined by his defeat at the hands of Jaghatai Khan, Mortarion abandons the pursuit of the White Scars and instead leads the Death Guard in a spiteful, punitive rampage across the systems of the Prosperine empire. World after world has fallen to this horrific onslaught, and yet the insular and secretive primarch seems preoccupied by some other, unspoken goal. Finally, on Terathalion, the truth of Mortarion's sinister heritage will be exposed, and the future of the XIV Legion will be written...

Read it because
The Death Guard have already embraced treachery, but this story follows their Primarch as he continues upon the road that will eventually doom his Legion to a plague-wracked damnation.