The Adeptus Mechanicus were’ split in 'two by the schism; over half of the Titan Legions turning their city-razing Weapons upon their fellows. At Tallarn, the Iron Warriors bombed the-verdant paradise world, stripping its surface to the arid, poisonous Wwastéland, it is today. At Logres, the Emperor's’ Children enslaved a million krill farmers: every single one died within a week as the Traitor Legionnaires sealed their pact with Slaanesh.
Horus and his hordes pushed on from sector to sector, inexorably fighting their way towards the Segmentum Solar. Recovering from their initial surprise, loyalist forces began to oppose the traitors. Spearheaded by loyal Space Marines and Titan Legions, the Emperor's forces contested world after world. Just as the loyalists began to make serious headway however, Horus demonstrated his strategic brilliance once more.
Horus had intended all along to strike a single, decisive deathblow. against the Imperium, and he meant to do so at Terra. When the attack came, the loyalists found themselves woefully unprepared: Horus’ cunning became even more apparent as the majority of the Emperor's Legions found themselves too far from Terra to intervene in time.
The Siege of the Emperor's Palace was the very darkest hour of Human history. The skies were turned black with Chaos drop pods and Dreadclaw Assault craft, and only three loyal Legions stood at the Emperor's side to oppose them.
The Emperor had always been guided by his innate prescience, but the future beyond this day was hidden from him. Then, at the very moment of the Warmaster's victory, Horus lowered the shields protecting his ship. Whether Horus was driven to do so by some last vestige of regret, or whether he simply wished to send his psychic sight to witness his father’s death will never be known, but in the instant the shields were lowered, the Emperor became aware of the traitor’s presence, and saw what must be done. Gathering his immediate companions, the Emperor, the Primarchs Sanguinius and Rogal Dorn, and a select group of warriors teleported directly to the Warmaster’s battle barge. The dropping of the shields allowed one last opportunity at salvation. The final, desperate gambit was made as time ran out for the Human race.
Upon Horus's ship the Emperor and his warriors found themselves separated, each forced to confront a sea of Warp-spawned horrors alone. At length, the Emperor fought his way to the bridge, only to find Horus standing over the broken body of his brother Sanguinius, Primarch of the Blood Angels Legion. Turning, Horus faced his creator, and in an instant the two were engaged in a battle likened to that between gods.
The Emperor triumphed. He siew his most beloved son, destroying utterly his presence in the Warp and casting his blasted form to the deck, But the Emperor had paid the highest price for his victory, and death overcame him as he finally allowed his body to suffer the terrible wounds Horus had inflicted.
The battle for Terra, the Imperium, and the fate of Mankind was won, but at a terrible cost. The Emperor’s wounds were so horrific that only his ascension to the arcane life-support systems of the specially constructed Golden Throne could hold his death at bay. Terra was a ruin; her cities levelled. So deep were her wounds that the tectonic plates groaned under the punishment inflicted by the traitors’ orbital bombardment. The routed hordes of Chaos had left such utter devastation in their wake that nothing short of the complete rebuilding of Terra, and of the Imperium itself, would even begin to heal the wounds inflicted during the Horus Heresy.
The _ Heresy may have failed in its objective of replacing the Emperor's rule with that of Horus, but it had brought the realm of man to within an inch of total annihilation. The Imperium was in tatters, and as the Traitor Legions retreated to the Eye of Terror, they knew that they would return, that they would rise again. Amongst them was Abaddon, Captain of the First Company of the Sons of Horus; he took with him the body of the Warmaster and an unquenchable appetite for vengeance. The Long War had begun, and the Traitors vowed they would wage it for all eternity to see their hatred satisfied.