THE HORUS HERESY
Ten thousand years ago, when the Imperium was young and the Emperor walked among men, rebellion flared. The Imperium was split and Mankind teetered on the edge of disaster. A new age, one ruled by the forces of Chaos, began in civil war and with acts of desperation and brutality. General Horus was regarded as the finest military commander of his age. When brave deeds were common, he was exceptional, and the Emperor eventually honoured him with the title of Imperial Warmaster, second only to the Emperor himself. Yet at the height of his powers, Horus, the hero who could do no wrong, turned against his Emperor. On the feral world of Davin, the Warmaster was possessed by a Daemon from the warp.
With a tool such as Horus, the servants of Chaos planned to destroy the Imperium and install their own puppet Emperor. Darkness would be unchecked and universally triumphant. The daemonic Horus, still respected and loved by his troops, plotted and conspired; all his skill was bent towards the destruction of the Imperium and his Emperor. The plot was eventually exposed, and thus began the First Inter-Legionary War.
The pax Imperia dissolved as news of the rebellion spread Many Imperial units declared for Horus, attracted by his reputation, yet unaware that he had been corrupted by Chaos. Old dissatisfactions with the Emperor’s rule were voiced once more, and battles erupted in system after system. For each regiment, corps and army that joined the rebellion there were those who remained loyal. The war and the Imperium hung in the balance, neither side able to deliver the crushing blow.
And then Horus struck at the heart of the Imperium. The Warmaster mounted a direct assault on Terra; the Luna defences were crushed, and the Imperial Palace itself breached. His followers, the Traitors of the Adeptus Astartes, Mechanicus and Titanicus, fought a savage campaign. No quarter was given or asked. All that stood between Horus and victory on Terra were the Adpetus Custodes, the Emperor’s personal bodyguard, the Imperial Fist and White Scars Marine Chapters and the Titan Order of the Fire Wasps. The struggle for the Imperium would be won and lost in the battle for the Imperial Palace.
And on the 55th day of the battle, the Traitor Legions and their Titan allies reached the walls of the Inner Palace. Despite acts of insane valour by the loyalists, victory was within Horus’ grasp. As the Outer Palace was abandoned to the rebels, the Emperor acted. An elite company of Adeptus Custodes and Imperial Fist Marines teleported into the Warmaster’s command bunker. They were lead by the Emperor himself A brutal firefight ensued; the Emperor sustained terrible wounds and Horus was killed. As word of the Warmaster's death spread, the Traitors paused, and then fell back to their transport barges. The Emperor was carried from the field and placed in a support bubble. In the confusion of victory and defeat, however, the Warmaster's body was never found. The Sons of Horus Chapter had fled from the Palace with his remains, the signal to all that the rebellion had failed.
The Traitors fled into the Eye of Terror, a collection of dust shrouded hell-worlds battered by warpstorms and worse. The Emperor was confined to the Golden Throne of Terra, the machine that has supported him to this day. He pronounced his judgement upon the rebels: they were to be exiled and accursed. All records were destroyed. It was to be as if the Traitors had never been part of humanity and the Imperium. Seven years of hard fighting remained, as the rest of the Imperium was cleansed of the naïve rebels who had sided with the Warmaster. All were destroyed or driven into the Eye during the Great Scouring.
The Traitors have remained sealed within the Eye of Terror. Lest they should return to threaten the good order of the Imperium, its boundaries are constantly patrolled and monitored. The Imperial Fleet and the Inquisition maintain a close watch upon the doings of the Traitors, while over one hundred Titan Orders of the Divisio Militaris are assigned to worlds near the Eye. They have no purpose but to stand ready against the day when the Traitor Legions and their Chaos-Titans could once again threaten the Imperium. For ten thousand years watchfulness around the Eye of Terror has been a constant duty, borne with quiet pride by the Emperor’s servants. It is the mark of their unshakeable loyalty to the Imperium and the person of the Emperor. A fuller account of the Horus Heresy, its aftermath and the participating Traitor Legions can be found in Realm of Chaos.