I have to say up front: I'm a huge fan of ADB. His contributions to the 40K universe are amongst my absolute favourite reads. So, when I learned about him writing a novel about the Eaters of Worlds, I was thrilled. And I for once was not disappointed by his work.
ADB has done a very fine job in depicting one of the most unrelatable Legions in a comprehensive and compelling picture. To me, the World Eaters always were borderline lunatics and butchers...even before the Heresy. But now you can understand how this Legion came to be a bunch of bloodthirsty psychopaths: because of their father.
While the other Primarchs have always been described as being benevolent towards their sons, Angron is the utter opposite. He does not care for his Legion, or the Crusade, or the Heresy, for that matter. He only cares for the nails in his head. And he ordered his sons to implant themselves with the same devices, shattering everything this once noble band of brothers once stood for.
As a result for that, he seems to be the only Primarch who is not loved by his own sons...but he doesn't care.
I really enjoyed how ADB paints a livid picture of a Legion in decay. Duty, Honour, Loyalty...everything starts to fall to the song of the nails, making the World Eaters difficult allies at best and mindless berzerkers at worst.
Another point worth mentioning is Khârn.
Personally, I always felt Khârn to be the least interesting character in the Codex Chaos of the 40K Tabletop-Game. Sure, he had a background story, but it always felt...dull. I mean, what do you expect from a character whose only goal in live is to kill anything he can?
But recently, the Black Library has published a number of stories which made me change my mind. Anthony Reynold's "Chosen of Khorne" and ADB's "Butchers Nails". If you really want to get an insight into the minds of Khârn, I strongly recommend to read to/listen to these works.
And what he started in "Butcher's Nails", ADB continues in "Betrayer". Khârn, at least pre-Heresy Khârn, finds himself between a rock and a hard place: either he disobeys a Primarch that he feels will destroy his Legion, or he gives in to the song of the nails.
I REALLY enjoyed learning more about this character.
My only grudge (albeit a small one) is that this novel is not entirely sef-sufficient. If you haven't read "Butcher's Nails" or "The First Heretic", you'll be more than a little confused at some points.
But all in all, I have to say: absolutely worth its money!