Chaos Space Marines

Chaos Space Marines, also known as the Traitor Legions, are Space Marines who betrayed the Imperium of Man to join Chaos. Most come from the Legions who sided with Horus during the Horus Heresy, though some later chapters of Loyalist Marines ended up joining Chaos as well. Because their technology is limited to what was available during the Horus Heresy, they are at a technological disadvantage to the loyalists. However, they are compensated by using blessings from the Chaos Gods, as well as access to heretical technologies such as Daemon engines.

The Chaos Space Marines are heavily divided, lacking the strict discipline they once had when serving under the Emperor. Many charismatic and ambitious warlords will lead a warband to pursue their own agenda, but many of them will unite together for a major conflict, especially one of the many Black Crusades.

Black Legion
The Black Legion was known as the Sons of Horus during the Horus Heresy, but after Horus' death, Abbadon took over and remade the legion into a powerhouse for Chaos veterans. While others pursue their own agendas or those of their patron god, the Black Legion is committed to finishing what Horus started and destroy the Imperium. The so-called Long War had lasted 10,000 years and has comprised of 13 Black Crusades, with the final one succeeding in destroying Cadia and allowing the Eye of Terror to expand.
 * Primarch: Horus
 * Notable members: Abaddon the Despoiler

Word Bearers
The Word Bearers are the most religious of the Chaos Space Marines, and worship all four gods as well as make ready use of daemons in their ranks. Their elites, the Gal Vorbak, willingly allow themselves to become possessed, while Dark Apostles maintain discipline by chanting from the Book of Lorgar and use sorcery to manipulate reality.
 * Primarch: Lorgar

Iron Warriors
The Iron Warriors are brutal and calculating, and are among the best engineers and heavy weapons experts among Traitor forces. They will often carve out their own mini-empires and view everything and everyone as a resource to be exploited. They view their own forces as mostly expendable, weeding out the weak and unworthy through scheming and suicidal assaults, whereas those who are more crafty can create extremely deadly traps for their enemies. They despise the Chaos Gods, but hate the Imperium even more.
 * Primarch: Perturabo

In TTS the Iron Warriors are just as childish and entitled as their Primarch. They’re frequently derided as trench-diggers and carry around shovels to knock over Imperial Fist sand castles.

Night Lords
The Night Lords terrorize their foes to such a degree that even rumor of their arrival will cause worlds to surrender. They will create fear and panic wherever they go, crafting brutal displays of violence and torture so that future victims take heed not to cross them. They also hate Chaos and care only for destruction.
 * Primarch: Konrad Curze

World Eaters
The World Eaters are murderous psychopaths that serve Khorne. Their Berserkers are often implanted with "butcher's nails" which effectively lobotomizes them to be unthinkingly aggressive. They mostly fight in close assault, wielding their signature chain axes.
 * Primarch: Angron
 * Notable members: Kharn the Betrayer

Thousand Sons
The Thousand Sons are lead by powerful sorcerers, with the rank-and-file Rubric Marines being little more than dust entombed in armor. They are the pawns of Tzeentch, and will often find that their own schemes are subordinate to those of their God. Many sorcerers operate in small secretive covens, performing various arcane rituals and devious plots.
 * Primarch: Magnus the Red
 * Notable members: Ahzek Ahriman

Death Guard
The Death Guard are extremely resilient to all damage, especially their Plague Marines. They are morbid and self-loathing to the point that they want to inflict as much suffering as possible to the rest of the galaxy, so they spread disease and decay in their wake in service to Nurgle.
 * Primarch: Mortarion
 * Notable members: Typhus

Emperor's Children
Once they were staunch perfectionists, but their obsessions lead them to pursue extensive body modifications and performance-enhancing drugs, until they became complete hedonists seeking their next high. They serve Slaanesh and specialize in sonic weaponry.
 * Primarch: Fulgrim
 * Notable members: Lucius the Eternal, Fabius Bile

Alpha Legion
This mysterious legion is known little of, even by other Chaos forces. They specialize in misinformation and employ a massive spy network. Rarely do they ever act openly, and almost never in large forces. A common trait is that any Alpha Legionnaire could impersonate Alpharius Omegon, and they still claim to be Alpharius on occasion.
 * Primarch: Alpharius Omegon

Warbands
Many smaller war bands have formed since the heresy that are either offshoots of the broken Traitor legions, or new bands of Chaos Space Marines brought together by the same goals. They frequently lack the full resources that the original legions had, but can still cause a lot of trouble for any world they strike.

Fallen Angels
The Fallen Angels were members of the Dark Angels who stayed behind on Caliban during the Horus Heresy. Their leader, Luther, was unhappy with his position and sought to free Caliban from the Imperium, and stumbled upon Chaos in the process. Not all of the Fallen followed him, though, with some believing that they had been betrayed by Lion El'Jonson but still refuse to follow Chaos. Nonetheless, they are hunted by the Dark Angels to keep their existence a secret.
 * Notable members: Cypher