In the time of the Horus Heresy, nearly half of the Space Marine Legions deserted the Emperor to follow Warmaster Horus. Nine of the twenty Legions that were in existence at that time rebelled and became the Traitor Legions. Each of the Traitor Legions has its own unique character and fighting stlye, but they are all united in their hatred of the Imperium and their thirst for revenge for the defeat they suffered ten millenia ago.
The Death Guard
During the Horus Heresy, the Death Guard joined Warmaster Horus in many battles and raids on the Imperium. When Horus led his forces against Earth and the Emperor, the Death Guard became lost in the Warp. While they were trapped in the Warp, a strange and deadly infection started to spread amongst the Legion, spreading from ship to ship. The stinking pestulence flooded the gut and disteanded the flesh and rotted its victims from the inside out. Even the Legion's Primarch, Mortarion, became infected, and in his delirium he called upon the powers of Chaos to aid the Space Marines. His fevered ravings were answered by Nurgle, and he became Nurgle's champion.
After Horus's defeat, Mortarion led his Death Guard in a campaign of destruction over a score of planets, until finally retreating into the Eye of Terror. Here he recieved Nurgle's ultimate reward, and became a full-fledged Daemon Prince, ruling over one of Nurgle's greatest Plague Worlds in the Eye of Terror. Mortarion sends out fleets of Plague Ships into the Warp to carry their contagions throughout the galaxy.
The Word Bearers
Even before Horus had been corrupted, Lorgar, Primarch of the Word Bearers, began to worhip the gods of Chaos. He revelled in the different aspects of each of the dark powers, dedicating himself to Chaos in its purest form, as Chaos Undivided, and he quickly led the Word Bearers along the same path. The fanatical zeal the Word Bearers had shown in their worship of the Emperor was quickly diverted into equally fanatical devotion toward Chaos.
The Word Bearers are the only Chaos Space Marine Legion of the original nine to still have Chaplains, who enforce a strict regime of rligious observance upon their Brothers. Word Bearers are zealous in the extreme, marching forward under huge banners dedicated to Chaos in its myriad forms.
The Night Lords
Their Primarch, Konrad Curze, called the Night Haunter, knew only one way, that was the use of viscious force. His methods were simple, viscious and direct, if you broke the law you died, there was no appeal, Night Haunter was judge, jury, and executioner upon the world Nostrama, where he had set himself up as a vigilante against the crime lords of the planet. When the Great Crusade finally reached this dark world, and the Emperor was reunited with this dark-visaged young Primarch, Night Haunter was placed in command of the Night Lords. The Night Lords quickly gained a reputation for ruthless efficiency, and a cynical disregard for human life. As long as they achieved their objectives, the means didn't matter to them. When the Emperor recalled Night Haunter to answer the charges of cruelty and destruction against him and his men, Night Haunter quickly defected to the side of Warmaster Horus. Night Haunter, operating from a planet deep in the widerness area of space known as the Eastern Fringe, led the Night Lords on a campaign of terror and genocide that has rarely, if ever, been equalled. Even after Horus had been defeated, the Night Lords continued their attacks until finally the Imperial Assassin M'Shen was able to infiltrate Night Haunter's base and slay the Primarch, with this act the Night Lords quickly stopped being an organised threat to the Imperium.
The New Lords now strike from the Eye of Terror, where they have retreated to, and they fight for the pure pleasure of it, and for the material rewards it can bring, and not because they worship some deity. They loo down upon the more dedicated Chaos Marines, and Loyal Marines, considering them fools.
The Alpha Legion
The Alpha Legion was the last Legion creatd during the First Founding, it being the 20th Legion. When Horus made his pact with Chaos, the marshall pride of the Alpha Legion was their downfall. The Warmaster was a mighty warrior himself, he commanded armies and fleets, and commanded at the forefront of the Emperor's wars. By comparison, he made the distant Emperor on Terra seem weak and cowardly. The Alpha Legion continues to fight a covert war against the Imperium to this day. They are connected with Chaos Cultist of many of the settled worlds of the Imperium. The Alpha Legion directs the activities of Cultists across entire sectors and they instigat massive insurrections against Imperial rule.
The Emperor's Children
The Emperor's Children were among the units assigned to crush Horus and his rebel Legions on Istbaan V. During a parly, the Legion's Primarch, Fulgrim, and his highest ranking officers were corrupted by the decadent pastimes Horus and his Chaos worshippers offered. The Imperial Cult was quickly supplanted by the gratifying worship of Slaanesh.
While corrupt beyond human comprehension, the Emperor's Children are a savage fighting force. Like many of Slaanesh's followers, they have become known as Noise Marines. Each suit of armour, evry bolter or chainsword is worked into fantastic patterns and coloured in praise of Slaanesh.
The Iron Warriors
The Iron Warriors once formed the Emperor's most able body of siege troops. The Iron Warriors' Primarch, Purturabo, excelled in siege and warfare above all else. Wherever the Iron Warriors fight they throw up great evil citadels in their wake, and hold them against all comers. Fields of trenches and forests of razorwire surround the strongholds of the Iron Warriors. Even after Horus's defeat, the Iron Warriors were only driven out of the Imperium worlds at a terrible price.
The Thousand Sons
Their Primarch is Magnus the Red, called the Red Cyclops or Cyclopean Magnus, due to his one large eye and his flaming-red hair. Magnus had already been touched by Chaos long before the Horus Heresy, from his long study of the Arcane arts. Even though the Thousand Sons tried to use their occult powers to warn the Emperor of Horus's heresy, the Emperor, mistrustful of anything tinged by Chaos, declared the Thousand Sons heretics and sent Leman Russ and the Space Wolves to devastate the Thousand Sons' homeworld of Prospero. The Thousand Sons were driven to war against their Emperor, and had to fight alongside the Traitor Legions for their survival. The Thousand Sons turned to Tzeentch and asked for his patronage, as she is the greatest wielder of Magic among the Chaos gods.
Magnus the Red was elevated to the rank of Daemon Prince of Tzeentch and given a Daemon World to rule over. He rules from his great volcanic fortress called The Tower of The Cyclops. The topmost level of the fortress has a single living eye, which watches over the landscape and the minions of its Lord, The Eternal Watchdog of The Sorcerer King. Over time, the Thousand Sons started to degenerate and become mutated. The Sorcerers of the Thousand Sons joined together in a cabal led by Ahriman. Even though they risked the wrath of their Daemon Primarch, they cast the spell The Rubric of Ahriman, purging the Thousand Sons of mutation for all of eternity. This caused the great schism within the Legion, and caused the cabal to be banished by the enraged Primarch and scattered to fight in different Traitor Legions across The Eye of Terror.
The Black Legion
The Black Legion began life as The Luna Wolves, created during The First Founding. The Emperor later changed its name to The Sons of Horus, in honour of its Primarch, in recognition of his and its many accomplishments in The Ullanor Crusade.
It was as The Sons of Horus that the Legion fought in the Horus Heresy, serving as Horus's Praetorians throughout his campaign. They were the first to remove the Imperial Eagle from their armour and Banners, and replaced it with The Eye of Horus.
Factions of the other Traitor Legions later blamed The Sons of Horus for causing the rout from Earth by retreating into Warpspace with the body of their beloved Warmaster.
The Sons of Horus at first worshipped one Chaos power after another, more and more of their number becoming possessed by Daemons. Over the centuries, they were also thinned out by their constant wars with the other Traitor Legions and Chaos forces. When Abbadon came to power in the Legion as their new Warmaster, his first edicts rejected the name of Horus and their ancient Legion title. He ordered the remaining members of the Legion to repaint their armour black in eternal memory of their shame at allowing the body of the Warmaster to be taken. He then led then in a lightning raid which destroyed the Warmaster's body and all of his clones. The Legion's remaining battle barge acts as a secret base for Abaddon and his Black Legion from which they make raids into the Imperium and The Eye of Terror.
The Worldeaters
The Worldeaters were easily converted to following the Chaos god Khorne due to their marshall pride and their belief that The Emperor and The Imperium had become soft and decadent. They have totally devoted themselves to fighting as close combat troops, feeding upon the berzerk rage of Khorne and distaining ranged weapons as the weapons of weaklings. They arm themselves entirely with pistols and close combat weapons, huge chainaxes and chainswords becoming their favoured tools of bloodshed.
The Worldeaters have split into separate squads, each following its own Champion of Khorne. They have become so engrossed in their fighting rage that any chance for them to operate as a cohesive Legion is long past.