Perturabo – master of siegecraft, and the executioner of Olympia. Long has he lived in the shadow of his more favoured primarch brothers, frustrated by the mundane and ignominious duties which regularly fall to his Legion. When Fulgrim offers him the chance to lead an expedition in search of ancient and destructive xenos weaponry, the Iron Warriors and the Emperor’s Children unite and venture deep into the heart of the great star maelstrom that haunts Perturabo's dreams. Pursued by vengeful survivors from Isstvan V and the revenants of a dead eldar world, they must work quickly if they are to unleash the devastating power of the Angel Exterminatus!
There are only a handful of must read books in the Horus Heresy series. This is one of them. It has mystery, things blowing up, backstabbing, honor, xenos and more...
The title is stupid
The term "Angel Exterminatus" is stupid and shouldn't have happened, that said the book is pretty good provided you can ignore the idiot title(which is used in the book), and it gives some insight into the cracks that were forming between the traitor legions even during the Horus Heresy, creating infighting where the Loyalists were fighting as a unified force for the most part