Warhammer 40k Lore: Where To Start -- The Gods of Chaos
In the Warp exist four powerful beings whose existence is derived from the psychic energy of all living creatures in the universe. (This doesn't refer to psykers exclusively, but to the mental and emotional power inherent in every being.) Therefore, the Chaos Gods, or Dark Gods, are forever tied to those living in the Materium, or the tangible non-Warp part of the universe.
The four Chaos Gods are Khorne, Tzeentch, Nurgle, and Slaanesh. Each of the gods takes some portion of their power and characteristics from specific emotions and actions reflected in the living world.
The Chaos Gods do not act together as a single group, and often hate and fight each other.
Where Did The Chaos Gods Come From?
Because the Chaos Gods derive their power from the emotions and mental power of living creatures, they didn't exist from the beginning of time--they only came to be after there was enough life in the world to support their creation. As more creatures evolved, their power grew.
Khorne was the first Chaos God to awake, during 1st or 2nd Millennium. Tzeentch, came shortly thereafter, along with Nurgle. Slaanesh didn't appear until the 29th millennium, as a direct result of the Fall of the Eldar.
Who Are The Chaos Gods and What Are Their Characteristics?
Khorne: The Blood God
Khorne is the Chaos God whose power is derived from hate and anger. Because of his rise during the Middle Ages of Terra, those years were particularly violent and war-prone. Although some of his followers believe that violence is justified through honor and bravery, Khorne does not share this belief--he only cares that violence is carried out, however it happens.
the warcry of Khorne is "Blood for the Blood God! Skulls for the Skull Throne!" and indeed Khorne is often shown sitting on a throne upon a massive pile of skulls--everything from human to Tyrranid skulls, anything killed by a Khorne champion.
Khorne is associated with the number eight, and it appears in the makeup of his armies, the structure of his Brass Citadel, and the various rituals and practices of his followers.
Khorne's greater daemons are Bloodthirsters, which resemble classic demons with cloven feet, wolf-like faces, and giant wings. The common daemons are Bloodletters, which are semi-humanoid creatures who have a crazed lust for violence and no thought for strategy. The daemon beasts of Khorne are Flesh Hounds, which hunt their cowardly prey by following their psychic energy. And the daemon steeds of Khorne are Juggernauts, which look roughly like a metal rhinoceros
The World Eaters, the traitorous 12th Legion of Space Marines, are dedicated to Khorne, and they specialize in close combat and feature the Berserkers, a crazed warrior who cares only about spilling blood, usually with a chainaxe or chainsword.
The Chaos God that Khorne hates most is Slaanesh.
Tzeentch: the Changer of Ways
Tzeentch is an interesting Chaos God, in that his specialty does not, at first, appear malicious: he is the god of change, and the instability that comes from change. The emotions that he feeds on are ambition and the desire for a different existence--even the desire for a better world. Because of this he is always scheming and always has plans and contingencies, and he can partially see the future because he knows the desires and plans of all mortals. He also the Chaos God most closely associated with magic.
Because of his nature, his appearance also changes frequently, his skin moving and undulating into different forms, including many different faces. He is sometimes represented as some form of massive snake, sometimes with many heads and faces, somewhat like a hydra.
The greater daemons of Tzeentch are the Lords of Change, which are a humanoid-bird-snake combination. They are very powerful with magic, but not as powerful in pure physical combat. The common daeemons of Tzeentch are Horrors, which, like Tzeentch, change their appearance continually.
Followers of Tzeentch will occasionally ride on Discs of Tzeentch, a magic metal platform that has a demon bound to to.
The Thousand Sons, the traitorous 15th Legion of Space Marines, are dedicated to Tzeentch, and are heavy magic users. The Thousand Sons underwent change, as was the will of Tzeentch, but when the sorceror Ahriman realized that Tzeentch was playfully corrupting and mutating the Thousand Sons, rather than protecting them, he cast a spell to stop the change and the spell left the Thousand Sons as near-souless suits of animated armor.
The Chaos God that Tzeentch hates most is Nurgle.
Nurgle: the Lord of Decay
The emotions that Nurgle is derived from are the fear of death, as well as insecurities and self-delusions. He grows stronger as corruption spreads, and he is often sought out by those who are sick or suffering, but instead of curing them, Nurgle makes them more corrupted--but content in their condition. They draw pleasure from their ruined state.
Nurgle himself is most often seen as an enormous and bloated mass of disease, replete with boils, maggot-filled sores, oozing wounds, and inhuman growths.
The greater daemons of Nurgle are Great Unclean Ones, which are very akin to Nurgle himself in their appearance: bloated, putrid and diseased. They carry a Plague Sword which is covered in contagion. The common daemons of Nurgle are Plaguebearers, who are humanoids with one eye who also carry rusty plague-spreading knives. Nurglings are tiny daeomons, who are almost always smiling and cheerful. They accompany many other units, but can also fight as a swarm.
The Death Guard, the traitorous 14th legion of Space Marines, are dedicated to Nurgle. During the Horus Heresy, the Death Guard were betrayed by corrupted leader, Typhon, who led them to a Destroyer Plague. Mortarion, the Primarch of the Death Guard came face-to-face with Nurgle, who told him that the Death Guard would suffer endlessly unless they pledged themselves to Nurgle. When they did, they were all corrupted and became the Plague Marines. (Typhon became Typhus, Herald of Nurgle.)
The Chaos God that Nurgle hates most is Tzeentch.
Slaanesh: The Prince of Pleasure
While the other Chaos Gods all appeared during Terra's Middle Ages, Slaanesh did not awake until the 29th millennium, at the Fall of the Eldar. The Eldar were prosperous and arguably the most powerful race in the galaxy and their power turned them to excess and pleasure-seeking. They became debauched, indulging in every vice and reveling in them all. All of these emotions--lust, greed, gluttony--fed into the Warp and formed Slaanesh. The birth of Slaanesh caused the opening of the Eye of Terror.
Slaanesh is typically depicted as female on the right side and male on the left, but can be male or female or asexual.
The greater daemon of Slaanesh is the Keeper of Secrets, which can look like anything from a brutal monster to the most beautiful person. The common daemons of Slaanesh are Daemonettes, which, like Slaanesh are often male on one side and female on the other and generally have claws. The Steeds of Slaanesh are snakelike, and ridden by Daemonettes.
The Emperor's Children, the 3rd Legion of the Space Marines, align themselves with Slaanesh. They delight in excess and use sonic weapons to demoralize the opposition.
Slaanesh doesn't have an intense hate for any of the other Chaos Gods, and is mainly neutral, but is generally opposed to Khorne.
Chaos Gods in Age of Sigmar and Warhammer Fantasy Battles
The four Chaos Gods also appear in Age of Sigmar and Warhammer Fantasy Battles, but their backstories are different from those in Warhammer 40k. In Warhammer Fantasy Battles, there is a similar concept of the Warp, but the Chaos Gods no longer existed in the Warp after the Coming of Chaos brought them into the Old World. In Age of Sigmar they exist the Realm of Chaos, which is very similar to the Warp: it is a realm of magic and emotions that is different from any of the eight Mortal Realmshperes. Obviously, in Warhammer Fantasy Battles and Age of Sigmar, the Chaos Gods are not related to the Terran Middle Ages or the Fall of the Eldar.
However, in Age of Sigmar and Warhammer Fantasy Battles there is an additional Chaos God, the Great Horned Rat. It is unclear how powerful he is compared to the other four, but he is powerful enough that he exists alongside them, even though they may look upon him in disdain.