Also this "The Imperium, but even more swarm-like and worse" is in a few of the novels in this series so maybe they all have the same origin. Mortarion: The Pale King addresses the first campaign that the newly-renamed Death Guard embark upon after being reunited with their Primarch.

I’m not sure if I’ve merely selected the wrong ones, or if maybe the questions I have about the primarchs are so niche and weird that the series doesn’t touch them. As well as establishing Mortarion’s character as a stoic puritan, it also hints at where we know his arc will go. Non-chronological telling of a brutal compliance (and Morty being knocked by Sanguinius and Horus for said brutality, which may or may not have been justified). The plot itself was compact, full of tension and pushed the narrative forward with every single page.Overall not a bad effort, but with too many fairly drab action scenes this one is probably only of real interest to Death Guard mega-fans. Maybe other approaches are better suited for planets those primarchs encounter, but he will deal with his obstacles his way.

Mortarion is no exception to this rule, and while the novel which bears his name has little to do with his upbringing, it does have a lot to say about his views on the Imperium. In a lot of ways, Mortarion is to Corax as Konrad is to Corax—a dark mirror lacking the optimism of the Raven Guard.This book comes close to flirting with some explanation as it’s revealed that while the Emperor understood Mortarion’s actions, he was deeply disappointed in him. Fresh from blowing the character studies of Vulkan and Guilliman, Annandale takes the chance to reflect.