Space Marine (1st ed, 1989), p7 — The Horus Heresy

General Horus was the finest military commander of his age. The Emperor granted him the title of Imperial Warmaster - a high honour, even in an age when brave deeds were commonplace. But the Imperium's hero was to become its deadliest enemy.

On the feral world of Davin, the Warmaster arranged to be inducted into a mystical warrior-lodge. This was not uncommon; Imperial policy approved of such links between feral-world natives and 'the Warriors from the Sky'. However, the warrior-lodge on Davin was not what it seemed. During the initiation, Horus became possessed by a Daemon from the warp. Those who had known Horus well began to speak of changes: lapses in memory, minor changes in habits, small things. Not until the Scouring of Isstvan did the truth become clear.

The Imperial Commander of Isstvan III had declared the system independent. Horus took five Space Marine Chapters to crush the rebellion: the World Eaters, Emperor's Children, Death Guards, Thousand Sons and his own Chapter, the Sons of Horus. He introduced the warrior-lodges to the Chapters, and the rot spread throughout his command. The Daemon-Horus planned to strike at the Imperium itself, and establish his rule throughout the galaxy. But first, Isstvan had to be dealt with.

The Scouring of Isstvan was a brutal affair. Isstvan III was destroyed by a virus bomb; twelve billion died instantly. It is said that the psychic death-scream of the doomed world drowned out the signal of the Astronomican.

During the bombardment, the frigate Eisenstein was seized by a small group of loyal Space Marine officers. They fought clear of the war-fleet and carried the news of Horus' corruption to Terra. Thus began the Horus Heresy wars.

The Imperium split almost evenly. The corruption of Horus' warrior-cult had spread from his command into several other units. Many more were attracted by his military reputation. Seven Space Marine Chapters - fully one-third of the Legiones Astartes - were sent against Horus. Four of these - the Word Bearers, Night Lords, Iron Warriors and Alpha Legion - joined the Rebels; the others have been purged from Imperial records.

While civil wars raged on a million worlds, Horus struck at Terra. The Adeptus Mechanicus workshops on Mars were quickly overrun, and as quickly besieged by the Loyalists. Of all the Titan Legions of the Divisio Militaris, only those on Terra remained loyal. Horus landed on Terra, and immediately lay siege to the Emperor's palace. On the 55th day of the battle, as the Inner Palace fell, the Emperor himself teleported into the Rebel command chamber with an elite force of Adeptus Custodes and Imperial Fist Marines. Horus was killed - although his body was never found - and the Emperor was so grievously wounded that he has been confined ever since to the biomechanical life-support unit known as the Golden Throne.

With Horus dead, the Rebels were thrown back from Terra. Soon the whole Sol system was recovered. But across the galaxy, the battles continue - the Rebels have been denied a swift victory, and the fate of Humanity still hangs in the balance. Throughout the Imperium there is war, the most dreadful war that Mankind has ever known. For the first time, Space Marines fight their fellow Marines - continents burn and planets are laid bare as the ultimate warriors meet in battle to decide the fate of all Mankind.