The Chaos Space Marines splintered away from the loyalist Space Marines during a major event called the Horus Heresy, which was a massive civil war within the Imperium of Man. This conflict occurred after Horus, the Emperor’s most trusted and capable Primarch (a genetically enhanced leader of the Space Marines), was corrupted by the powers of Chaos. The Chaos Gods (entities of the Warp, a parallel dimension of pure psychic energy) manipulated Horus, convincing him to rebel against the Emperor, promising him power and dominance over the galaxy.
At first, it wasn’t a simple matter of the Chaos Space Marines just disagreeing with the Emperor’s vision. Many of them were loyal to Horus and felt betrayed by the Emperor’s perceived neglect or favoritism. Some were drawn to the promises of power from Chaos, while others were slowly corrupted without fully understanding what they were getting into. Over time, Chaos twisted and mutated many of the Space Marines, making it difficult, if not impossible, for them to ever return to the Emperor’s service. So, while some made a conscious choice to rebel, many others were gradually corrupted beyond recognition.
Chaos Space Marines are incredibly capable and dangerous warriors. They started by taking Imperial technology with them when they turned traitor. This included tanks, warships, and other advanced war machinery. Some of these machines were corrupted by the power of Chaos, taking on twisted forms. Over the millennia, they’ve also developed new technology, often through dark rituals and the help of the Chaos Gods. They also raid the Imperium to steal supplies and tech, further supplementing their war efforts.
In the early days after the Horus Heresy, the Chaos Space Marines retreated into the Eye of Terror, a massive Warp storm where the powers of Chaos are strongest. From there, they launched raids and invasions against the Imperium. The war between the loyalist Space Marines and their corrupted counterparts has raged ever since, with the Chaos Space Marines always looking for ways to spread their dark influence and destroy the Emperor’s empire.
