Short answer
Every faction in Warhammer 40k falls into one of three camps. The Imperium is humanity's empire and its many armies. Chaos is everyone who serves the dark gods of the warp, including traitor Space Marines and daemons. Xenos covers the alien races, from Orks to Necrons. Within each camp, factions still fight each other all the time.
The Imperium: humanity's armies
The Imperium of Man rules about a million worlds in the name of the Emperor. It's huge, slow and cruel, but it's also the only thing standing between ordinary people and a galaxy that wants them dead. Its military is split into separate organizations that often don't get along.
The poster children are the Space Marines, genetically engineered warriors organized into Chapters. Many Chapters are different enough to field their own armies, such as the Blood Angels, Dark Angels, Space Wolves and Black Templars. Above them in prestige stand the Adeptus Custodes, the Emperor's own bodyguard.
Most of the actual fighting falls to the Astra Militarum, the Imperial Guard. These are normal humans with lasguns and tanks, thrown at problems in the millions. Beside them fight the faith-fueled Adepta Sororitas, the machine-worshipping Adeptus Mechanicus from Mars, the daemon-hunting Grey Knights and the towering Imperial Knights. Agents such as Inquisitors and assassins also turn up in many battles.
Chaos: the traitors and the daemons
Chaos factions serve, or are used by, the four Chaos Gods: Khorne, Nurgle, Tzeentch and Slaanesh. The core of Chaos is the traitor Space Marine Legions that rebelled against the Emperor during the Horus Heresy.
Some traitors serve no single god. The Chaos Space Marines army covers these, including the Black Legion, Night Lords, Iron Warriors, Word Bearers and Alpha Legion. Four Legions became so bound to one god that they field their own armies. The Death Guard belong to Nurgle. The Thousand Sons belong to Tzeentch. The World Eaters belong to Khorne. The Emperor's Children belong to Slaanesh.
Then there are the Chaos Daemons, beings made of warp energy who can only cross into reality in special conditions, and the Chaos Knights, giant war machines whose pilots turned to darkness.
Xenos: the aliens
"Xenos" is the Imperium's word for anything not human, and it doesn't mean the factions are allies. Most of them hate each other as much as they hate humans.
- Aeldari. Once ruled the galaxy. Their decadence helped birth the Chaos god Slaanesh, which wrecked their civilization. They now survive in scattered groups. A newer movement, the Ynnari, follows a god of the dead.
- Drukhari. The Aeldari who never gave up their cruelty. They hide in a city inside the webway and raid for captives.
- Orks. A fungus-based species that lives to fight. They're funny, chaotic and one of the biggest threats in the galaxy, simply because there are so many of them.
- Necrons. An ancient race who traded their bodies for metal ones and slept for sixty million years. Now they're waking up and want their worlds back.
- T'au Empire. A young race that believes in cooperation for the "Greater Good". They're more reasonable than most factions, though not as kind as they claim.
- Tyranids. Bio-engineered swarms from outside the galaxy, driven by a single hive mind. They consume every living thing on a planet.
- Genestealer Cults. Families of human hybrids who hide in plain sight and call the Tyranids down on their own world.
- Leagues of Votann. Short, tough and grown in clone vats. They're distantly descended from humans but have been cut off from the Imperium for millennia. They mine, trade and settle scores.
Every faction at a glance
This table lists the armies that have their own standalone rules as of the current edition. Some Space Marine Chapters, like the Blood Angels or Space Wolves, are played as Space Marine variants and aren't listed separately.
| Faction | Camp | What they want | How they feel |
|---|---|---|---|
| Space Marines | Imperium | Protect humanity and the Emperor's legacy | Elite super-soldiers. Few models, each one tough. |
| Adeptus Custodes | Imperium | Guard the Emperor and Terra | The Emperor's personal guard. Even fewer, even tougher. |
| Astra Militarum | Imperium | Hold the line by sheer numbers | Ordinary human soldiers, tanks and artillery. |
| Adepta Sororitas | Imperium | Fight in the Emperor's name as his faithful | Armored warrior nuns driven by faith. |
| Adeptus Mechanicus | Imperium | Recover lost technology and serve the Machine God | Cyborg tech priests and their augmented troops. |
| Grey Knights | Imperium | Hunt daemons | A secret Space Marine order of psychic warriors. |
| Imperial Knights | Imperium | Uphold noble oaths | Huge walking war machines piloted by knightly houses. |
| Chaos Space Marines | Chaos | Revenge on the Imperium, power from the gods | Traitor Space Marines, many from the Heresy itself. |
| Death Guard | Chaos | Spread Nurgle's plagues | Rotting, near-unkillable traitor marines. |
| Thousand Sons | Chaos | Hoard knowledge and sorcery for Tzeentch | Sorcerers leading cursed, dust-filled armor. |
| World Eaters | Chaos | Kill for Khorne | Berserk close combat traitors. |
| Emperor's Children | Chaos | Chase every sensation for Slaanesh | Excess and vanity turned into a war band. |
| Chaos Daemons | Chaos | Break into reality | The gods' servants made flesh. |
| Chaos Knights | Chaos | Rule by terror | Corrupted versions of the Imperial Knights. |
| Aeldari | Xenos | Survive and avoid their doom | An ancient, dying race of fast, precise warriors. |
| Drukhari | Xenos | Feed on suffering to stay alive | Dark kin of the Aeldari who raid from a hidden city. |
| Orks | Xenos | Have a good fight, forever | Loud, crude, endless green-skinned hordes. |
| Necrons | Xenos | Reclaim the galaxy they once ruled | Ancient robotic dynasties waking from a long sleep. |
| T'au Empire | Xenos | Spread the Greater Good | A young, optimistic empire with advanced guns. |
| Tyranids | Xenos | Eat everything | A hive mind swarm from beyond the galaxy. |
| Genestealer Cults | Xenos | Prepare worlds for the Tyranids | Hybrid cults hidden inside human society. |
| Leagues of Votann | Xenos | Profit, survival and old grudges | Hardy clone-born miners with heavy tech. |
Picking a side as a beginner
Start with the look. You'll spend hours building and painting, so choose models you enjoy looking at. Then read the lore of that faction and see if its outlook grabs you.
Some rough guidance. Space Marines are the easiest to learn because there's so much beginner material for them. Necrons and Death Guard are forgiving because their models are durable. Orks and Tyranids look fun but mean painting a lot of models. Aeldari and Drukhari reward careful play but punish mistakes.
If you want to understand why the sides hate each other, read about the Emperor and the Heresy first. Almost every human faction's story traces back to those two things.
Corin Ashdown is the pen name of the editor who writes and checks every guide on Vorbelutrioperbir. How we write and check guides.