Chaos Space Marines are Space Marines who have fallen and been corrupted by the powers of Chaos. Once loyal warriors of the Imperium, the Traitor Legions now bear a burning hatred for the "False Emperor", and all those who serve him.
In combat, Chaos Space Marines are brutal and efficient killing machines. Depending on which Chaos God they follow, they can be anything from maddened berserkers, to cold and calculating sorcerers, but all of them are dangerous opponents to face.
10,000 years before the present time, nine of the eighteen Space Marine Legions were coaxed to turn to Chaos by the favored Primarch and right hand of the Emperor, Warmaster Horus. This led to a massive civil war known as the Horus Heresy, where Astartes made war on each other for the first time. The Traitor Legions managed to reach and lay siege to Holy Terra itself, but in the end they were turned back, with Horus slain by the Emperor himself.
The loss of Horus caused the Traitor Legions to scatter, and they retreated to the Eye of Terror, a region in space where the Warp mingles with real space. From there they strike at the Imperium, disorganized, but full of hate, and sometimes possessing daemonic powers.
Because of time passing erratically within the Eye of Terror, some of the Chaos Space Marines encountered in M41 are the same people who fought in the Horus Heresy; for some of them, the Siege of Terra has just ended, while others have experienced ten thousand or more years of seemingly eternal war against the Loyalists and their False God. Other Astartes and sometimes entire Chapters have fallen at a later time.
Currently, the Traitor Legions are shattered and disorganized, with the Chaos Space Marines split up between a myriad of Warbands. Despite this, however, they are still a grave threat to the Imperium.
- Chaos Space Marine (campaign enemy)