The Horus Heresy, as detailed in books like "The Burden of Loyalty" and "The Chaplain," reshapes the Imperium. Loyalist legions, now led by figures such as Roboute Guilliman and Marneus Calgar, fight to preserve the Imperium against their traitor brethren. The war is brutal and devastating, with iconic battles on Terra itself, the capital world of the Imperium. The Siege of Terra, covered in novels like "The Siege of Terra: The Gate of Heaven," represents the climax of the conflict, as Horus and his forces attempt to overthrow the Emperor.
– Introduces the idealistic era of the Great Crusade, the Luna Wolves, and the elevation of Horus to Warmaster. Warhammer 40k - Horus Heresy - Books 1-54 -comp...
The Horus Heresy, as depicted in the first few books of the series, such as "Horus Rising" and "The False Gods," sets the stage for the catastrophe. Horus, once the most trusted of the Emperor's generals, becomes increasingly disillusioned with the Imperium and the Emperor's intentions. He is swayed by the Chaos gods, ancient and malevolent deities worshipped by the traitor legions. These gods promise Horus the power to save the galaxy from the Imperium's perceived shortcomings. The subtle corruption of Horus and several of the Primarchs by Chaos marks the beginning of the end. The Horus Heresy, as detailed in books like