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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 Sons of Horus 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 (Alpharius of the Alpha Legion and Night Haunter of the Night Lords). (Alpharius is believed to still be alive, possibly through his brother-primarch, 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.

The Chaos Space Marines have the same origins as the Space Marines, thus have the same physical power within the game. Due to their allegiance to Chaos, veteran Chaos Marines can be mutated, thus making them into monsters, faster or otherwise superior to other gaming units. Chaos Marines are also virtually immortal due to the time-warping effects of the Eye of Terror, whereby their millennia of experience afford them levels of mastery with advanced battlefield skills and tactics that younger Space Marines may not yet have. Chaos Marines have also lost the "they know no fear" ability. So unlike a Space Marine, a Chaos Marine may break and run away if they start to lose the fight.

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. While Chaos Space Marines have therefore mostly older technology, they compensate for this by having access to daemons and daemonic technology.

Each of the Chaos Space Marine Legions fights using a different style of warfare; also, four of the nine are dedicated to one of the four major Chaos Gods. Codex: Chaos Space Marines (Chambers et al, 2002) includes the current rules for fielding a Chaos Space Marine army in a game of Warhammer 40,000; it also includes rules for fielding the troops of the specific legions that are dedicated to any of the Chaos Gods, with rules for the other legions being removed in the 4th edition Codex.

The nine Chaos Marine Legions are (in order of founding) Emperor's Children, Iron Warriors, Night Lords, World Eaters, Death Guard, Thousand Sons, Black Legion, Word Bearers, Alpha Legion.

The Emperor's Children has a number of notable members, those include Lucius the Eternal, Saul Tarvitz, Fabius Bile, Gaius Caphen, and Charmosian. The legions are often featured in short stories and novels. For example, The Night Lords appeared in the short story Chains of Command. Another appearance is made in the Space Wolves story "Sons of Fenris", where a group of Night Lords are fighting against Wolf Lord Ragnar Blackmane and his cadre of Wolfguard. They also appeared in the Eldar stronghold mission of Warhammer 40,000: Dawn of War: Dark Crusade (it is said that they were lured by the Eldar as a distraction). The Iron Warriors have appeared in the novel "storm of Iron" and favour siege warfare.

In addition, since the Horus Heresy, several Space Marine Chapters have been corrupted in one way or another by Chaos. Very little background information has been provided about these Chapters, and no specific rules exist for them, allowing players to adapt other rules to represent their forces.

According to the records of Ordo Malleus approximately 50 chapters have turned renegade after the Horus Heresy, however the accuracy of this number is doubtful. Examples of given space marine chapters include the Sons of Malice, the Damned Company of Lord Caustos, the Violators, the Steel Cobras, the Thunder Barons, and the Astral Claws (known as the Red Corsairs since their rebellion).

• The death metal band Debauchery (band) 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.

space marines are really really really lame

• WH40K Wiki Chaos Space Marine
• Chaos Weapons, Equipment and Vehicles
• Vehicles of the Imperium (Warhammer 40,000)
• Vehicles of the Space Marines (Warhammer 40,000)