"Death to the False Emperor! Death to the weakling Imperium of Mankind!" —Oath of the Traitor Legions

A Chaos Space Marine, also sometimes called a Traitor Marine, a Renegade Marine, or a Heretic Astartes, is a former Loyalist Space Marine of the Imperium of Man who has chosen to abandon the service of the Emperor of Mankind and dedicate himself to the service of Chaos to achieve his own ends. Such corrupt Astartes normally belong to one of the nine original 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 that have turned Renegade. 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 and pledged their souls to the service of the four Ruinous Powers of Chaos. Some Traitor Marines pledged their service to all four of the Dark Gods, a force known as Chaos Undivided, whilst others preferred to dedicate themselves to the worship of only one of the Ruinous Powers. 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 and ambition.

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 potent infantry and combat walker forces of 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, mayhem, 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.

When an Astartes falls from grace, he will fall further than other mortals. A lifetime dedicated to self-deprivation and selflessness is simply thrown aside and the Space Marine indulges his newfound independence in thought and deed to its fullest extent. Freed from the constraints of the discipline, traditions and responsibilities that define the Adeptus Astartes, a Chaos Space Marine can exercise his every whim, whether it be for bloodshed, mayhem, sensual pleasures long denied, ambition, love or simple vengeance. Given full reign to his long-supressed personal desires, his incredible physical powers and mental strength can be turned to the pursuit of the most malevolent acts, untrammeled by any sense of either mercy or guilt. When an Astartes' will finally breaks, the result is catastrophic as the whole edifice of his purpose and psyche tumble into an often violent maelstrom of unleashed and pent-up desire..