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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.

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 Legions of Space Marines who fought in the Great Crusade for the Imperium of Man. At this time the Warmaster Horus, first among the Primarchs, and the Luna Wolves (later 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 ten 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, legendary 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 seven surviving Primarchs have since become Daemon Princes. These daemonic Primarchs rarely take part in the affairs of their Legions or any part of the  epicWarhammer 40,000 universe. For example, Magnus the Red, Primarch of the Thousand Sons is the most powerful of all the Daemon Prince sorcerers. Yet he rarely enters the battlefield, even though prior to the Horus Heresy he always took part in his Legion's battles and was a determined warrior. Angron however, Primarch of the World Eaters, led an invasion force in what was to become the First War of Armageddon, where he was banished for 100 years by a group of 100 Grey Knights led 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.

Gods of Chaos
Khorne: Khorne is the Chaos God of war, blood, and violence, and personifies the emotions of hatred and blinding rage. He acts outwardly by seeking the deaths of others, preferring close combat over ranged weaponry, and the only things he respects are strength and martial prowess. Khorne's followers are always ferocious warriors. Men turn to Khorne for martial power and the strength to conquer but he doesn't care for his followers or from where the blood flows, only that it does as he sits upon a throne crafted from the multitudes of skulls from enemies and allies alike. He openly despises magic in all its forms, believing it to be effeminate, weak, and the craft of cowards.

Nurgle: Nurgle is the Chaos god of death and mortality, embodying disease and rot, and personifies the emotion of despair. His power comes from the inevitability of death and decay, and Nurgle is often referred to as 'Grandfather Nurgle', as entropy is the most ancient of forces, and he is the only one who even pretends to care for his followers. Nurgle's followers are granted power by their acceptance of this reality. Nurgle prides himself on the achievements of his followers, gifting them with hideous diseases while sheltering them from pain, and his followers rejoice in their blessings, shrugging off lethality and disfigurement in a state of rapturous undeath. Tzeentch is his equal and opposite, and the two will frequently battle each other.

Tzeentch: Tzeentch is the Chaos god of change, revolution, knowledge, vigour, and sorcery, and is the personification of hope or ambition. His titles include the Changer of Ways, the Architect of Fate and the Great Sorceror. Tzeentch excels in subtle machinations and is patron to schemers and conspirators of all sorts, favouring the cunning and the wise over the strong. He is also among the most progressive and enlightened of the Chaos gods, seeking betterment and refinement, though he ultimately manipulates his followers and betrays those who become useless to him.

Slaanesh: Slaanesh is the Chaos god of lust, pleasure, and satisfaction, and personifies desire. A sensuous, androgynous deity that is simultaneously referred to as male and female, Slaanesh is associated with hedonism and decadence. He/she is known by many colourful names such as The Prince of Pleasure, She Who Thirsts, and the Lord of Excess. Slaanesh is said to be both the youngest and weakest of the four Chaos gods, though early in its existence it was far stronger, and murdered and consumed nearly the entire pantheon of Eldar gods. Slaanesh's creation was due to the sum of the Eldar's own excesses over millennia, due to their uniquely powerful psychic presence in the Warp, excesses described as equal parts hedonism and painful delight. This event led directly to the near-extinction of the Eldar, and the creation of the Eye of Terror. Mortals who seek charisma and fellowship follow Slaanesh, for its mark makes one popular and inspiring, though its followers are just as likely to be terrifying orgiastic maniacs. For this reason, the followers of Slaanesh are known to be particularly dangerous, as they seek pleasure with hopeless abandon in all martial actions, even those actions considered suicidal by any sane commander.

Although those are the main gods of chaos, there are many more. Some haven't even been referred to. These are just the four Ruinous Powers. Also, these gods have their equal and opposite, and frequently battle each other. Nurgle and Tzeentch oppose each other, as Nurgle embodies the static & inevitibility of death while Tzeentch is the personification of change & hope. Devotees of Slaanesh and Khorne frequently battle each other, since Khorne exists solely for the glory of combat - followers of Khorne favour might and brute strength over all, while Slaanesh exists solely for pleasure - The followers of Slaanesh embody excess in every way.

Forces of the Chaos Space Marines
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 (Thorpe, Cavatore et al., 2007) includes the current rules for fielding a Chaos Space Marine army in a game of Warhammer 40,000, 4th or 5th Edition. The previous Codex, published in 2003 for 3rd Edition, also includes rules for fielding the troops of the specific legions, but this Codex was designed for 3rd Edition and cannot be used with 5th Edition. Chaos space marines
 * Emperor's Children, devoted to the Chaos god Slaanesh
 * Iron Warriors, specialists in siege warfare
 * Night Lords, specialists in fast strikes and terror tactics
 * World Eaters, close combat specialists, devoted to the Chaos god Khorne
 * Death Guard, devoted to the Chaos god Nurgle
 * Thousand Sons, containing many sorcerers, devoted to the Chaos god Tzeentch
 * Black Legion, formerly the Luna Wolves and later Sons of Horus, now led by Horus' lieutenant Abaddon the Despoiler
 * Word Bearers, fanatical Chaos cultists who make extensive use of summoned daemons
 * Alpha Legion, specialists in infiltration and subterfuge
 * Red Corsairs, a pirate band specialists in hit and run attacks led by Huron Blackheart

The legions are often featured in short stories and novels. For example, The Night Lords appeared in the short story Chains of Command and also "Lord of the Night", a story that follows the Raptor lord Zso Sahaal in his struggle to regain his dead Primarch's stolen legacy, adding an entirely different view and depth to a legion that had previously little attention. 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. No specific rules exist for renegade chapters, allowing players to adapt other rules to represent their forces. According to the records of the Ordo Malleus (the body responsible for protecting the Imperium from Chaos and daemons), 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, the Sons of Vengeance, the Silver Guards and most notably the Astral Claws (known as the Red Corsairs since their rebellion).Their numbers are even harder to gauge, as usually when a chapter succumbs to Chaos there will be marines who remain loyal to the Emperor. One example is the Battle-Captain of the Death Guard's 7th Company, Nathaniel Garro, who rallied his company and escaped with other surviving loyalists from Istvaan V, returning to Terra and warning them of Horus's betrayal. Another example is Iacton Qruze, the only member of the Sons of Horus to remain loyal to the Emperor and survive the Horus Heresy (both of these Marines are rumoured to have formed the core of what would become the Imperial Inquisition). These loyalist survivors are rare, as they are usually outnumbered and slaughtered by their corrupted brethren. However, the few loyalists that do survive when a chapter falls are amongst the most skilled warriors in the Imperium.

It is also worth note that the Thousand Sons didn't fall from the path of the Emperor and were still loyal, and in fact tried to warn the Emperor of Horus's upcoming treachery. However the methods used in the divination and form of the warning is what angered the Emperor and they were called to Terra to answer for themselves, also it was Magnus's intervention that prevented the Emperor from accessing the Eldar webway through the use of the Golden Throne therefore removing humanities reliance on the warp for transport. It was only the interception, and subtle twisting of this order by Horus that set the Space Wolves on an Exterminatus rather than an escort mission to Prospero. Even in these extreme circumstances the Thousand Sons were still loyal, and thought the Wolves the traitors, and fled Imperial space to avoid persecution. Only then with their already close connection to the Warp, and the damage done by the Failure of the Rubric of Ahriman, did they turn from the Imperium.

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 (except for the Thousand Sons Rubric Marines who are now empty suits of walking armor with the souls of their owners trapped within because of The Rubric of Ahriman, other Thousand Sons have shown signs of mutation), 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 the fact that space marines as a whole are unable to die of old age, 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. However, the reason that they haven't yet caused any major hit to the Imperium is their own anarchy. If they were to organize and form an army equivalent to the tactical army of the Imperium, then they would be a much graver threat than they are now. That is what Abaddon is trying to do: to unite all of the Chaos forces from the tiniest warband to the greatest legions and lead them to a last Black Crusade against the Imperium by attacking on Terra once more.

Chaos Marines 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 or traded to them by black market merchants; 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 or rediscovers technology designed during the "Golden Age of Technology" (equally well known as the "Dark 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 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 Millennium", and "Through The Eye Of Terror".
 * American Doom Metal band Cirith Ungol released the songs "Chaos Rising" and "Join the Legion" on their 1991 album Paradise Lost.