In the table-top wargame Warhammer 40,000, the Chaos Space Marines or Chaos Marines, are Space Marines who serve the Chaos Gods. They are also referred to as the Traitor Legions, primarily in background material written from the perspective of the Imperium.
The background shown in both Codex: Space Marines (Haines and McNeill, 2004) and Codex: Chaos Space Marines (Chambers et al., 2002) states that the Chaos Marine Legions were nine of the twenty original "First Founding" Legions of Space Marines who fought in the Great Crusade for the Imperium of Man. At this time the Primarch Horus and the Luna Wolves were corrupted by Chaos and instigated the galaxy-wide civil war known as the Horus Heresy.
Further background to the Chaos Space Marines is explored in detail in the 'Horus Heresy' series. After the death of Horus and the end of the Heresy, the remnants of the nine Legions along with the other Imperial forces that had joined Horus escaped into an area of the galaxy known as the Eye of Terror. Due to the nature of Chaos, and the temporal instability of the Warp, the very same Chaos Marines who revolted against the Emperor continue to fight against the Imperium.
The Legions have kept their old names, with the exception of the Sons of Horus who were renamed the Black Legion by their new leader, Abaddon the Despoiler. Besides Horus, two other Chaos Primarchs were believed to have been killed during or shortly after the Heresy (Konrad Curze of the Night Lords and Alpharius of the Alpha Legion though Alpharius is believed to still be alive, possibly through his Twin Brother, Omegon) The six surviving Primarchs have since become Daemon Princes. These daemonic Primarchs rarely take part in the affairs of their Legions or any part of the Warhammer 40,000 universe.For example, Magnus the red, the most powerful of all the daemon prince sorcerers, does rarely enter the battle field, even though when he was a pure space marine, he always took part in his legions battles and was a determined warrior. Angron however, led an invasion force which was to become the First War of Armageddon, where he was banished for 100 years by a group of 100 Grey Knights lead by Brother-Captain Aurellian. Some of the Legions have pledged a particular loyalty to one of the four Great Chaos powers. The closest thing to a leader that has appeared among them is Abaddon the Despoiler. Abaddon is the only Chaos Marine since Horus to be able to command the loyalty of all nine Traitor Legions, and has led thirteen Black Crusades against the Imperium of Man.
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