Chaos Space Marines

A Chaos Space Marine, also sometimes called a Traitor Marine, is a former Loyalist Space Marine of the Imperium of Man who has chosen to betray the service of the Emperor of Mankind and dedicate his soul to the service of Chaos and its foul deities. Such corrupt Astartes normally belong to one of the 9 Traitor Legions that betrayed the Emperor during the ancient civil war called the Horus Heresy more than 10,000 standard years ago, while others come from Space Marine Chapters created long after the Heresy ended. Chaos can corrupt Astartes of any time and place just as its siren call leads many lesser men to their damnation.

During the Horus Heresy, the terrible Imperial civil war that pitted the Battle-Brothers of the 18 known Space Marine Legions against one another, there were two factions. These were the Loyalist Space Marines who continued to serve the Imperium and the Emperor of Mankind and the Chaos Space Marines of the Traitor Legions who betrayed the Emperor's trust and pledged their souls to the dark service of the 4 Ruinous Powers of Chaos. The Traitor Legions were led by the Warmaster Horus, once a Primarch of the Imperium and the Emperor's most trusted friend, son, and adviser before he was corrupted by the promises and lies of the Dark Gods and his own inner flaws.

The Traitor Legions were the most powerful component of the Forces of Chaos which fought during the Heresy, which also included the myriad daemons of the Warp, human Traitors from the Imperial Army, the Dark Mechanicus and Chaos Cultists drawn from across the galaxy. The flesh and Power Armour of Chaos Space Marines often warps and twists into new, darker and inhuman forms under the influence of the Chaos energies they are exposed to as their shapes morph to embody the corrupted Astartes' new allegiance. Chaos Space Marines fight for everything they were once forbidden to indulge in as the ascetic servants of the Emperor and the selfless guardians of Mankind. Pleasure, wealth, but above all else the power to rule their fellow men in the name of the Chaos Gods, is the driving ambition and goal of every Chaos Space Marine.

History
The Traitor Legions of the Chaos Space Marines represent 9 of the 20 original First Founding Legions of Space Marines who were created by the Emperor of Mankind from the genomes of his 20 Primarchs in the late 30th Millennium to fight the Great Crusade that forged the Imperium of Man. At this time Horus, the Warmaster of the Imperial forces and Primarch of the XVI Legion, the Luna Wolves, (which the Emperor allowed him to rename the Sons of Horus in his honour before the onset of the Horus Heresy to demonstrate his status as first-among-equals among the other Primarchs), was corrupted by Chaos and instigated the galaxy-wide Imperial civil war known as the Horus Heresy in his determination to replace the rule of the Emperor with his own.

After the death of Horus at the hands of the Emperor aboard his Battle Barge Vengeful Spirit during the Battle of Terra that ended the Horus Heresy, the remnants of the 9 Traitor Legions fled, along with the other Traitor Imperial forces that now served Chaos, into an area of the galaxy where the Warp bled into realspace, creating the permanent Warp Storm known as the Eye of Terror. Due to the nature of Chaos, and the temporal instability of the Warp, many of the very same Chaos Space Marines who revolted against the Emperor more than 10 millennia ago continue to fight against the Imperium today, having effectively been granted a tortured immortality by the will of the Ruinous Powers.

The Traitor 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, once Horus' chief lieutenant and the First Captain of the Luna Wolves before he followed Horus into damnation. Besides Horus, 2 other Traitor Primarchs were killed during or shortly after the Heresy (Alpharius of the Alpha Legion and Konrad Curze of the Night Lords). The 6 surviving Traitor Primarchs have since been transformed into Daemon Princes by the Chaos Gods they serve. These daemonic Primarchs rarely take part in the affairs of their Legions or are major players in the affairs of the galaxy. Four of the Traitor Legions have pledged a singular loyalty to one of the great Chaos Gods. while the others serve the interests of Chaos as a whole in the form of Chaos Undivided. Due to the nature of Chaos and those who choose to serve it, not long after the end of the Heresy the Traitor Legions, save for the Word Bearers, largely stopped operating as unified military organisation and instead devolved into a series of separate and often competing warbands. The Traitor Marines who command these warbands, all potent Chaos Champions, sought their own personal glorification and the eventual opportunity to become Daemon Princes in their own right by earning victories in the cause of Chaos.

Due to the fractious nature of the Dark Gods, the warbands in service to a particular Ruinous Power often view other Traitor Marine warbands who serve its patron's rival deity as even more appropriate targets for their wrath than the servants of the Emperor. With such an attitude prevailing among the corrupted Astartes, it has often proven impossible for the Traitor Legions to present a united front against the Imperium since the death of Horus, once reason why they have never enjoyed a similar series of military successes over the past 10 millennia. The closest thing to unity that has appeared among the Traitor Legions and other Chaos Space Marines is when Abaddon the Despoiler, the successor of Horus as the Warmaster of the Black Legion, launches one of his periodic Black Crusades into Imperial space under the banner of Chaos Undivided, which is a tempoary period of unity amongst all the Forces of Chaos when they seek to overwhelm the Imperium's defenses surrounding the Eye of Terror and drive on Terra once more. Abaddon is the only Champion of Chaos since Horus himself who is able to command, however begrudingly and briefly, the loyalty of all 9 Traitor Legions, and he has led 13 Black Crusades out of the Eye of Terror against the Imperium of Man. The latest, the 13th Black Crusade, was launched in 999.M41 and has proven the most successful for the Forces of Chaos since the Horus Heresy. The Chaotic troops were able to seize a foothold on the surface of the Fortress World of Cadia, though the Imperial Navy has so far managed to blockade Abaddon's Traitor Marines and the other Forces of Chaos from breaking out into the Cadian Gate, the only clear pathway through the Warp from the Eye of Terror into open Imperial space. Should the Chaotics manage to eventually break out of the Imperial Navy's stranglehold, the Chaos Space Marines may at last, after 10,000 years, bring down the realm of the Corpse Emperor and gain their vengeance for long millennia of exile. Then the galaxy will tremble as the true lords of Chaos rule!

Chaos and Loyalist Space Marine Differences
The Chaos Space Marines have the same genetic origins as the Space Marines in the gene-seed created by the Emperor from the genomes of the 20 Primarchs, and thus have the same physical abilities as their uncorrupted counterparts. Due to their allegiance to Chaos, Veteran Chaos Space Marines often suffer from extreme mutations (or Chaotic "gifts" depending on the point of view). Traitor Marines are virtually immortal due to their sojourns within the Warp, whereby their millennia of experience afford them levels of tactical mastery and advanced battlefield skills that the usually much younger Loyalist Space Marines may not yet have earned.

Chaos Space Marines are equipped with the ancient patterns of Power Armour and weapons they had when they initially betrayed the Imperium at the start of the Horus Heresy, which are broadly the same as those used by the current Loyalist Space Marines (although some differences now exist with regard to Jump Packs, Power Armour appearance, etc). At present in the late 41st Millennium, some 10,000 standard years after the Horus Heresy, the Imperium of Man has made a few, mostly minuscule technological advancements, due to the recovery of certain Standard Template Construct (STC) technologies from the interstellar human civilisation that existed during the Dark Age of Technology, though in most ways the Imperium has either completely stagnated or even regressed from the technological level prevalent across the Imperium during the Great Crusade in the early 31st Millennium. Chaos Space Marines have far more limited access to the handful of new inventions that have appeared on the galactic scene in the last 10,000 years. In general, anything developed since the Heresy is unavailable to the Traitor Marines unless it has been captured or was already deployed among Space Marines who went Renegade in more recent centuries; for example, the Iron Warriors Traitor Legion is well-known for capturing and using more modern patterns of Imperial tanks and armoured vehicles. While Chaos Space Marines may possess older technology than their Loyalist counterparts, they compensate for this weakness through the ability to summon daemons from the Immaterium and make use of daemonic technology like daemon engines, as well as the unbridled power of arcane sorcery drawn from the Warp without fears of limitation or corruption.

Because the Chaos Space Marines have all dedicated their souls to Chaos, they are extremely fearful of death, knowing that their souls will ultimately be consumed by the Ruinous Powers, much as Slaanesh does to the souls of the Eldar, unless they can earn the rare promotion to become Daemon Princes in their own right. The Chaos Gods do not truly care for their Chaos Space Marine servants, any more than they do for any mortals who serve them as mere pawns and will gladly see them tortured for all eternity within their realms in the Immaterium simply for their own pleasure. Worshipers of Khorne are cursed to feel eternal rage but denined the ability to slaughter, the children of Nurgle are forever consumed by despair and the wracking pains of a hideous death, those who serve Slaanesh as their patron will be tortured for the sheer pleasure of the Prince of Chaos until the end of time, and the servants of Tzeentch will know only endless insanity as their souls undergo a constant change in form and the knowledge of the universe they crave is forever placed just out of their reach. In theory, those souls who choose to serve Chaos Undivided are either shared or fought over by the Chaos Gods in the afterlife of the Immaterium. Only the promise of eternal life as a Daemon Prince, if that dubious honour can be earned through unbridled success in the service of Chaos, reassures those mortals who pledge their lives and souls to Chaos. This fear of death, however, does nothing to slow or halt the vicious might of the Chaos Space Marine armies and warbands, who are quite sure that the power they have received from the Chaos Gods in return for their souls will ultimately provide them not only with rule over their fellow men, as is the right of every Astartes, but with the opportunity to earn the necessary glories required to appease the Dark Gods and claim the daemonic immortality they offer as the ultimate reward.

Chaos Space Marine Traitor Legions
Each of the 9 Chaos Space Marine Traitor Legions fights using a different style of warfare that is defined by their Legion's culture and nature; also, 4 of the 9 -- the Emperor's Children, the World Eaters, the Thousand Sons and the Death Guard -- are dedicated specifically to the service of 1 of the 4 major Chaos Gods, Slaanesh, Khorne, Tzeentch and Nurgle, respectively. The other 5 Traitor Legions essentially serve the interests of all of the Ruinous Powers collectively in the form of Chaos Undivided.

The 9 Chaos Space Marine Traitor Legions are (in order of their Founding):

Renegade Space Marine Chapters
There are also many normal humans and mutants who serve the Ruinous Powers as Chaos Cultists or belong to ancient regiments of the Imperial Army or the Dark Mechanicus who joined Horus in his rebellion and then fled with the Traitor Legions into the Eye of Terror 10 millennia ago after Horus' defeat. These groups are often used by the Chaos Space Marines to bolster their ranks. These Traitors, mutants and heretics are collectively labelled the "Lost and the Damned" by the Inquisition. In addition, since the Horus Heresy, some Second Founding Space Marine Chapters have been corrupted in one way or another by Chaos and have become Renegade Chapters or Renegade warbands of Chaos Space Marines. According to the current records of the Inquisition's Ordo Malleus, approximately 50 Loyalist Chapters or elements of Loyalist Chapters have turned Renegade in the ten millennia since the Horus Heresy. Examples include the Sons of Malice, the Damned Company of Lord Caustos, the Violators, the Thunder Barons, and the Astral Claws (known as the Red Corsairs since their rebellion against the Imperium during the Badab War). A more complete list can be found here.

The Forces of Chaos
The Forces of Chaos is an all-encompassing term that includes the Chaos Space Marines and all the other beings and entities who serve the Ruinous Powers of Chaos and make war upon the Imperium of Man and the many other intelligent races of the Milky Way Galaxy. Among their number are included the Traitor Legions and Renegade Space Marines, human, mutant and alien Chaos Cultists, Traitor Imperial Guard, Traitor Titan Legions, the Tech-Priests of the Dark Mechanicus, Chaos Spawn cannon fodder, and daemons of every shape and level of power as well as countless other foul beings too horrific to be mentioned here. All of these forces can be led into battle by the Chaos Space Marines, who are collectively the most powerful mortal servants of the Ruinous Powers who have ever lived.