Further background to the Chaos Space Marines is explored in detail in the 'Horus Heresy' series. After the death of Horus and the end of the Heresy, the remnants of the nine Legions along with the other Imperial forces that had joined Horus escaped into an area of the galaxy known as the Eye of Terror. Due to the nature of Chaos, and the temporal instability of the Warp, the very same Chaos Marines who revolted against the Emperor continue to fight against the Imperium.
The Legions have kept their old names, with the exception of the Sons of Horus who were renamed the Black Legion by their new leader, Abaddon the Despoiler. Besides Horus, two other Chaos Primarchs were believed to have been killed during or shortly after the Heresy. The six surviving Primarchs have since become Daemon Princes. These daemonic Primarchs rarely take part in the affairs of their Legions or any part of the Warhammer 40,000 universe.
Chaos gods:
Khorne: Khorne is the Chaos God of war, blood, and violence, and personifies the emotion of hatred. He acts outwardly by seeking the deaths of others, preferring close combat over ranged weaponry, and the only things he respects are strength and martial prowess. Khorne's followers are always ferocious warriors.Nurgle: Nurgle is the Chaos god of death and mortality, embodying disease and rot, and personifies the emotion of despair. His power comes from the inevitability of death and decay, and Nurgle is often referred to as 'Grandfather Nurgle', as entropy is the most ancient of forces.
Tzeentch: Tzeentch is the Chaos god of change, revolution, knowledge, vigour, and sorcery, and is the personification of hope or ambition. His titles include the Changer of Ways, the Architect of Fate and the Great Sorceror. Tzeentch excels in subtle machinations and is patron to schemers and conspirators of all sorts, favouring the cunning and the wise over the strong.
Slaanesh: Slaanesh is the Chaos god of lust, pleasure, and satisfaction, and personifies desire. A sensuous, androgynous deity associated with hedonism and decadence. Slaanesh is said to be both the youngest and weakest of the four Chaos gods, though early in its existence it was far stronger, and murdered and consumed nearly the entire pantheon of Eldar gods.
Forces of the chaos:
Emperor's Children, devoted to the Chaos god Slaanesh.
Night Lords, specialists in fast strikes and terror tactics.
World Eaters, close combat specialists, devoted to the Chaos god Khorne.
Death Guard, devoted to the Chaos god Nurgle.
Thousand Sons, containing many sorcerers, devoted to the Chaos god Tzeentch.
Black Legion, formerly the Luna Wolves and later Sons of Horus, now led by Horus' lieutenant Abaddon the Despoiler.
Word Bearers, fanatical Chaos cultists who make extensive use of summoned daemons.
Alpha Legion, specialists in infiltration and subterfuge.
Red Corsairs, a pirate band specialists in hit and run attacks led by Huron Blackheart.
Night Lords, specialists in fast strikes and terror tactics.
World Eaters, close combat specialists, devoted to the Chaos god Khorne.
Death Guard, devoted to the Chaos god Nurgle.
Thousand Sons, containing many sorcerers, devoted to the Chaos god Tzeentch.
Black Legion, formerly the Luna Wolves and later Sons of Horus, now led by Horus' lieutenant Abaddon the Despoiler.
Word Bearers, fanatical Chaos cultists who make extensive use of summoned daemons.
Alpha Legion, specialists in infiltration and subterfuge.
Red Corsairs, a pirate band specialists in hit and run attacks led by Huron Blackheart.