Chaos Space Marines

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.

History
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. 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.

Differences between Chaos Space Marines and Space Marines
The Chaos Space Marines have the same origins as the Space Marines. Due to their allegiance to Chaos, Chaos Marines can be mutated or willingly possessed, thus making themselves into monsters, faster or otherwise superior to the Space Marines and other inhabitants of the Warhammer 40k universe. Chaos Marines have extended lifespans due to the time-warping effects of the Eye of Terror, and their millennia of experience gives them levels of mastery with more advanced skills and tactics that Loyalist Space Marines do not have. Regular Chaos Marines have also lost the "and they shall know no fear" ability. However, Regular Chaos Marines are still extremely hard to break due to their newfound loyalty in the pantheon of chaos, and some of the more veteran or bloodthirsty Chaos Marines are more fearless than their loyalist brethren.

They are equipped with the power armour and weapons they had when they initially betrayed the Imperium, which are broadly the same as those used by Space Marines (although differences now exist with jump packs etc). The current setting of the Warhammer 40,000 game is about 10,000 years after the Horus Heresy, and, while the Imperium has made some technological advancements, Chaos Marines have far more limited access to the handful of new inventions that have appeared on the galactic scene. In general, anything developed since the Heresy is unavailable to them unless it has been captured; for example, Iron Warriors are known for capturing and using Imperial tanks. This is not a strong differentiating factor though, as the Imperium merely replicates technology designed during the 'Golden Age of Technology' and has very little understanding of it; thus there have been very few developments during the last 10,000 years. While Chaos Space Marines have mostly older technology, they compensate for this by having access to daemons and daemonic technology.

Other tie-ins

 * The death metal band Debauchery released several tracks/albums with references to the World Eaters. Most notable is the track "KILL MAIM BURN!".
 * The British death metal band Bolt Thrower released a song titled "World Eater" on their album Realm of Chaos. The entire album is, in fact, themed around the Warhammer 40,000 concept of Chaos, as evidenced by the Games Workshop-produced artwork of the original printing, and song titles such as "Plague Bearer", "Dark Millenium", and "Through The Eye Of Terror".