Ten thousand years ago, Horus, Warmaster of the Imperium and most trusted 'son' of the Emperor, fell to Chaos due to a conspiracy by the Word Bearers. Besides the Space Marine Legion under his direct command, the Sons of Horus, eight of his brother-Primarchs and their legions also joined him in the Horus Heresy against the Emperor.
Only when Horus was slain at the ultimate conflict of the Heresy, the Battle of Terra, was the rebellion broken and the defeated Traitor Legions hounded into the Eye of Terror, which remains both their prison and sanctuary to this day. The Chaos Space Marines have not forgotten their defeat in the Heresy, and for the last ten thousand years have waged the Long War 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.) 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 Warhammer 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. Some of the Legions have pledged a particular loyalty to one of the four Great Chaos powers but the eight Blood Lords are loyal to Angron alone. The same is the case with the Death Guard, where the Primarch Mortarion's war council is known as the Lords of Decay. Abaddon the Despoiler has taken charge of the Son of Horus, now known as the Black Legion. 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.
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. Some Traitor Marines pledged their service to all 4 of the Dark Gods, a force known as Chaos Undivided, whilst others preferred to dedicate themselves to the worship of only a single one of the Ruinous Powers. 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 Sorcerer. 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 and inevitability of death while Tzeentch is the personification of change and 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. When the devotees of Chaos die, their psychic energy, their souls in the Warp, do not fade away to an unknown destination, are not destroyed and are not devoured by daemons like the souls of others. Instead their souls are swallowed by the collective power of the gods, sustaining them and increasing the eternal power of Chaos. Technically the Chaos Space Marine Legions are the only Space Marine Legions still in existence, the loyalist Legions having been reformed into self-contained Chapters after the Heresy. Though the Legion remains alive to the Chaos Marines as an identifying concept, in reality all of the Chaos Legions have over time been scattered into separate war bands following rival Chaos Champions. On occasion a highly favoured Champion of Chaos, such as Abaddon, is able to temporarily reunite these Traitor Legions against the Imperium.
Emperor's Children, devoted to the Chaos God Slaanesh. The Emperor's Children were considered the most disciplined Legion before the Heresy, however their obsession with attaining perfection inevitably led to their downfall and worship of Slaanesh. The Emperor's Children make extensive use of Noise Marines, armed with exotic Sonic Weaponry. Iron Warriors, specialists in siege warfare. The Iron Warriors grew disillusioned during the Great Crusade, believing they had been betrayed and forced to fight endless sieges while the more favored Legions claimed honor and glory. They are the mortal enemies of the Imperial Fists, who similarly specialize in siege warfare. The Iron Warriors possess a large number of Obliterators, marines who have been turned into hulking monstrosities of flesh and machinery, able to create nearly any weapon from their bodies. Night Lords, specialists in fast strikes and terror tactics. Made up of criminals and murderers from their home world of Nostramo, the Night Lords were the most terrifying of the Legions. They were despised by their own Primarch Konrad Curze, a fact they were all unaware of. Kurze allowed himself to be killed by an Imperial assassin, wanting to prove that even the Emperor was willing to resort to such dishonorable actions. World Eaters, close combat specialists, devoted to the Chaos God Khorne. The most barbaric of the Legions, the World Eaters were known to slaughter countless in their fury. The legion's unity was shattered by their Champion Kharn the Betrayer who attacked his comrades in a rage, leading to infighting that caused them to split into smaller warbands. The World Eaters are devoted to the God Khorne, boasting more Khornate Bezerkers then any other legion. Death Guard, specialists in biological warfare, devoted to the Chaos God Nurgle. The Death Guard were known for their incredible fortitude, fighting across the most noxious and hazardous war zones unscathed. On their way to join the traitors besigeing Terra the Death Guard became trapped in the Warp where they were ravaged by plagues of the Chaos God Nurgle. In desperation their Primarch Mortarion swore devotion to the plague god to save his legion. The Death Guard is composed largely of Plague Marines, specializing in biological warfare. Thousand Sons, containing many sorcerers, devoted to the Chaos God Tzeentch. They are the sworn enemies of the Space Wolves, who even before their fall despised them for their use of psykic powers. During the Great Crusade the Thousand Sons devoted themselves to the pursuit of knowledge. In an attempt to warn the Emperor of Horus's betrayal the Legion's Primarch Magnus the Red resorted to forbidden sorcery. His plea was ignored and their rivals the Space Wolves were sent to apprehend him. Horus manipulated their orders causing the Space Wolves to assault and destroy the planet. The Thousands Sons were forced to swear loyalty to Tzeentch in order to survive and escape into the warp. There they would lick their wounds and regroup, so that one day they would enact retribution upon the Space Wolves Legion and their Primarch Leman Russ. In an attempt to stop the mutations plaguing them, the Legions sorcerers inadvertently turned all the non-psychic marines to dust, trapping their souls in their armor. While a very small warband the Thousand Sons have more Chaos Sorcerers then any other. Black Legion, formerly the Luna Wolves and later Sons of Horus, now led by Horus' lieutenant Abaddon the Despoiler. Before their fall the Luna Wolves and their Primarch were the most favored of the Space Marine Legions. Later renamed the Sons of Horus in their Primarch's honor, and the Black Legion after his death. Their leader Abaddon seeks to unite all the forces of chaos under him and destroy the Imperium. The Black Legion has launched thirteen of these Black Crusades. The Black Legion is the largest and most infamous of the Chaos Warbands. Word Bearers, fanatical Chaos cultists, utterly devoted to the dark gods, and the first legion to be corrupted. The Primarch Lorgar, leader of a religious order on his homeworld, grew disillusioned with the 'Imperial Truth', the rejection of religious superstition and belief in science and logic. Seeking something to believe in they turned to the worship of the chaos gods. The Word Bearers make extensive use of summoned daemons. Alpha Legion, specialists in infiltration and subterfuge. The last legion to be founded, the Alpha Legion was known for its use of deception and subterfuge to achieve its goals. Virtually nothing is known of the Alpha Legion. They spread lies and dissent where ever they go, fanning the flames of rebellion. Unknown to everyone, the Legion's Primarch Alpharius had an identical twin brother Omegon, who would often impersonate him while he performed secret tasks. The Alpha Legion makes extensive use of Chaos Cultists. Red Corsairs, a pirate band specialists in hit and run attacks led by Huron Blackheart. Prior to their fall the Astral Claws chapter led several other chapters to rebel against the rulers of the Imperium in the infamous Badab War. Following their defeat and the near death of their leader the Astral Claws fled into the Warp where they became the Red Corsairs. The Red Corsairs have amassed a massive pirate fleet, becoming a force to be reckoned with, rivaling that of the ancient traitor legions. Since the Horus Heresy, many 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).