Chaos Space Marines

The Chaos Space Marines are one of the forces of Chaos.

According to the story, 10,000 years ago, Warmaster Horus, Primarch of the Luna Wolves (later Sons of Horus, and the Black Legion) was tainted by Chaos while he recovered from wounds on the planet Davin, having been inducted into a "warrior lodge" that was little more than a Chaos cult. Horus introduced the "warrior lodge" ideal to five of the Chapters under his command, and spread to half of the twenty Legions. The corrupted ones wholeheartedly followed Horus's idea of a new Imperium of Man, with Horus as Emperor.

Unaware of the treachery, the Emperor sent Horus to Istvaan III, where the Imperial governor had declared the system independent of the Empire, to suppress the rebellion. Horus used a bioweapon that killed the population of 12 billion in seconds; releasing a psychic shockwave said to have been louder than the Astronomican.

The Emperor ordered the Inquisition to purge the Adeptus Astartes of the Chaos taint, and thus began the Horus Heresy. Nine Legions of Space Marines betrayed the Empire for Horus, who had destroyed three others. The Emperor, Primarch Sanguinius of the Blood Angels, and Primarch Rogal Dorn of the Imperial Fists attacked Horus on his ship. Horus slew Sanguinius when he refused to betray the Emperor, and the Emperor slew Horus in battle. Mortally wounded, the Emperor ordered his fleet to drive the Traitor Legions into the Eye of Terror, a rift in space where reality and the Warp merge.

The Traitor Legions and their Primarchs are:
 * Sons of Horus - Warmaster Horus
 * Thousand Sons - Magnus the Red
 * World Eaters - Angron
 * Night Lords - Konrad Kurze (Night Haunter)
 * Word Bearers - Lorgar
 * Alpha Legion - Alpharius
 * Iron Warriors - Perturabo
 * Death Guard - Mortarion
 * Emperor's Children - Fulgrim

The current leader of the Chaos Marines is Abaddon the Despoiler, the "son" of Warmaster Horus, who has led 13 Black Crusades against the Imperium of Man.

There are also many humans who serve Chaos - traitors, mutants and heretics. These scum of the universe are collectively labelled The Lost and the Damned.