Chaos Space Marines, also sometimes called Traitor Marines, Renegade Marines, or Heretic Astartes, are former Space Marines of the Imperium of Man who have chosen to abandon the service of the Emperor of Mankind and dedicate themselves to Chaos to achieve their own ends.

Such corrupt Astartes usually 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 and women 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.

A Chaos Space Marine of the Word Bearers Traitor Legion, the first of the ancient Space Marine Legions to turn to Chaos before the Horus Heresy.

Some Traitor Marines pledged their service to all four of the Dark Gods, a malevolent 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.

https://warhammer40k.fandom.com/wiki/Chaos_Space_Marines