Book 15 of "The Horus Heresy: Primarchs". Fantastic character focussed novel featuring the Primarch of the Death Guard legion.

Once, the Galaspar System suffered under the cruel regime of the Order. Billions of people toiled endlessly to enrich their masters, enduring short lives of poverty, squalor and fear. But Galaspar’s sins did not go unnoticed by the Imperium, and so Death itself sentenced the Order to annihilation. Mortarion, newly uplifted to commander of the Death Guard, descended upon the world, and with him came a slaughter of untold proportions.

The sheer brutality of Mortarion’s campaign left the Imperium appalled. Seeking to understand its horrors, two noble primarchs have come to Galaspar, summoning their brother to account for his actions. But the Pale King brooks no challenge to his methods, for when the scythe falls, it reaps a gruesome toll.