Ten thousand years ago the Imperium was racked by a terrible civil war called the Horus Heresy. The rebellion was led by Warmaster Horus, the most trusted of the Emperor's Primarchs, who had been led to believe by the Chaos Gods that it was he, not the Emperor, who should be the leader of humanity. Almost half of the Space Marine Legions of that time sided with Horus, plunging the Imperium into a terrible galaxy-wide war from which it has never fully recovered. The war ended with the death of Horus at the hands of the Emperor, but the Emperor himself was mortally wounded and is now only barely kept alive by a life-support unit called the Golden Throne. With the defeat of Horus, the surviving Space Marines that had followed him fled to the Eye of Terror - the greatest warp storm in the galaxy. The Chaos Space Marines of the Traitor Legions, as they were now called, lurk there to this day, consumed by their hatred of the Emperor, the Imperium and all Mankind. Twisted by continual contact with the Chaos Gods, Chaos Space Marines have become a terrible canker that eats at the heart of the Imperium and they might, one day, have their revenge... Taken from Codex: Chaos Space Marines