Betrayer by Aaron Dembski-Bowden

Set during the Horus Heresy, the novel follows a brutal encounter between two Traitor Legions—the ritualistic Word Bearers and the berserk World Eaters—on a devastated world, where worship, fury, and loyalties collide. Through visceral battles and close-point-of-view moments, commanders and veterans are forced to confront fanaticism, the corrosive effects of rage and faith, and the personal betrayals that fracture brotherhoods; the story probes how religious zeal and uncontrollable violence can each corrupt men and legions, leaving devastation in their wake.