Chaos Marines

Official Text
Legions of the Dark Gods

''From the depths of the Warp, the traitor Chaos Space Marine Legions emerge to wage war against the Imperium. Slaves to darkness, they seek to bring down the Imperium of Man and destroy all they once swore to protect.''

''The Black Legion is the largest and most powerful of the Traitor Legions, and the Iron Warriors are siege masters of the highest caliber. The Word Bearers are fanatical zealots of the Chaos, while the cruel Night Lords strike from the shadow, using fear as a weapon.''

While it is in the nature of the various Legions of Chaos to be duplicitous and self-serving, when they put aside their internecine struggles and combine their strength, the universe trembles.

Origin
The Chaos Space Marines, or Chaos Marines, are legions of Marines who follow Chaos after the Horus Heresy. Originating from the same ranks as the Space Marines, the Chaos Marines betrayed the Imperium of Man under leadership of Warmaster Horus, first amongst the Primarchs. With this betrayal, a galaxy-wide civil war was launched between the nine legions of the Imperium who turned to Chaos and the remaining eleven legions still loyal to the God-Emperor.

After the death of Horus and the end of the Heresy, the Chaos Remnant and the imperial forces who joined them disappeared into an area of the galaxy known as the Eye of Terror, allowing them to remain hidden and launch guerrilla campaigns against Imperial holdings. The Chaos Marines have kept their old Imperium names by and large, with the exception of the Sons of Horus, who were renamed the Black Legion by their commander, Abaddon the Despoiler.

Culture
Chaos Marine society hinges on the worship of the Chaos Gods and the brutality of warfare. Mirroring the Space Marines of the Imperium, Chaos Marines adhere to strict social and military orders, functioning almost in a caste system. Stripped of their humanity, Chaos Marines seek only to massacre their enemies and cause chaos and destruction.

Military
The Chaos Marines were originally made of the nine rebellious Horus Heresy legions, and were joined by additional Imperium troops during the revolution. Following their exile into the Eye of Chaos, their numbers have waned somewhat, but the time warping of the Eye and the already near-immortal nature of the Space Marines have led the Chaos Marines to be highly trained, specialized, and effective in the killing arts.

Allegiance
The Chaos Marines have but one allegiance - allegiance to the Gods Chaos and their emissary to all Chaos Marines, Warmaster Horus and his descendants in power. The Chaos Marines follow a Pantheon of Chaos Gods, detailed as such:

Khorne: Khorne is the Chaos God of war, blood, and violence, and personifies the emotions of hatred and blinding rage. 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. Men turn to Khorne for martial power and the strength to conquer but he doesn't care for his followers or from where the blood flows, only that it does as he sits upon a throne crafted from the multitudes of skulls from enemies and allies alike. He openly despises magic in all its forms, believing it to be effeminate, weak, and the craft of cowards.

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, and he is the only one who even pretends to care for his followers. Nurgle's followers are granted power by their acceptance of this reality. Nurgle prides himself on the achievements of his followers, gifting them with hideous diseases while sheltering them from pain, and his followers rejoice in their blessings, shrugging off lethality and disfigurement in a state of rapturous undeath. Tzeentch is his equal and opposite, and the two will frequently battle each other.

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. He is also among the most progressive and enlightened of the Chaos gods, seeking betterment and refinement, though he ultimately manipulates his followers and betrays those who become useless to him.

Slaanesh: Slaanesh is the Chaos god of lust, pleasure, and satisfaction, and personifies desire. A sensuous, androgynous deity that is simultaneously referred to as male and female, Slaanesh is associated with hedonism and decadence. He/she is known by many colourful names such as The Prince of Pleasure, She Who Thirsts, and the Lord of Excess. 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. Slaanesh's creation was due to the sum of the Eldar's own excesses over millennia, due to their uniquely powerful psychic presence in the Warp, excesses described as equal parts hedonism and painful delight. This event led directly to the near-extinction of the Eldar, and the creation of the Eye of Terror. Mortals who seek charisma and fellowship follow Slaanesh, for its mark makes one popular and inspiring, though its followers are just as likely to be terrifying orgiastic maniacs. For this reason, the followers of Slaanesh are known to be particularly dangerous, as they seek pleasure with hopeless abandon in all martial actions, even those actions considered suicidal by any sane commander.

Although those are the main gods of chaos, there are many more. Some haven't even been referred to. These are just the four Ruinous Powers. Also, these gods have their equal and opposite, and frequently battle each other. Nurgle and Tzeentch oppose each other, as Nurgle embodies the static and inevitability of death while Tzeentch is the personification of change and hope. Devotees of Slaanesh and Khorne frequently battle each other, since Khorne exists solely for the glory of combat – followers of Khorne favour might and brute strength over all, while Slaanesh exists solely for pleasure - The followers of Slaanesh embody excess in every way.