Book 3 in The Horus Heresy: The Siege of Terra.
The war for the fate of mankind blazes on. Though the outer defences have fallen, the walls of the Palace itself remain inviolate as Rogal Dorn, the Praetorian of Terra himself, uses every known stratagem and ploy to keep Horus's vast armies at bay. In Perturabo, the Traitor siegebreaker, Dorn faces an adversary worthy of his skill. A terrible, grinding attrition ensues. The crucial battle for the Lion's Gate spaceport is at the heart of this conflict. With it in their possession, the Traitors can land their most devastating weapons on Terran soil. Dorn knows it must not fall. But with enemies attacking from within as well as without and the stirrings of the neverborn drawn to the slaughter, can the Imperial defenders possibly prevail?