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Chaos Space Marines (also called Traitor Marines, Chaos Marines, and Traitor Legionnaires) is a word used to describe the many heretical Space Marines that betrayed the Emperor became servants of the Chaos Gods. They come in many forms and tend to have cults that are devoted to certain chaos deities.
Unlike many other factions within Warhammmer 40,000, rather than being united under the umbrella of a higher faction or as loosely affiliated groups, they are fractious: refractory and prone to infighting amongst themselves. The are only truly unified in their hatred of the Emperor Of Man and their desire to help Chaos become the ruling power of creation via acts of unspeakable evil.
They are a playable army choice in Warhammer 40,000 and also serve in a highly antagonisic role in the related media. Chaos Space Marines are the rough equivalent of Warhammer Fantasy's Hordes Of Chaos.
Chaos Space Marines draw their origins back to the the crusade to unite the Galaxy under one banner, called the Great Crusade. The Chaos Gods, perhaps fearing the eventual result, stopped their own conflicts to interfere with the works of the Emperor of Man.
Starting with the Emperor's disenfranchised son, the pious Lorgar Aurellian, the Chaos Gods seduced Lorgar into Chaos-worship by showing him horrifying visions of the future under the Emperor, and convinced him that without the aid of Chaos humanity would share the fate of the Eldar. Having won over a powerful leader and orator into their cause, the Chaos Gods' campaign to subvert the Emperor's faithful soldiers began.
After purging his Legion of any elements with any loyalties left to the Emperor, Lorgar sent his First Captain Erebus as an ambassador. Erebus, keeping his Legion's new faith a close secret, began organizing the Warrior Lodges, secretive fraternal orders within the Space Marine Legions that existed outside of the Legion's regimented and ordered strictures. Though these were under the guise of providing brotherhood without rank, they were instrumental in fomenting sympathies and paving the way for the coming betrayal.
Erebus proved instrumental for setting the stage to the Horus Heresy after seeding the Warrior Lodges into a full half of the Legions and successfully engineering the fall of Horus into Chaos.