In Games Workshop's Warhammer 40000 and Warhammer Fantasy fictional universes, Chaos refers to the often malevolent entities which live in The Warp, composed mostly of Daemons, but also includes those mortals who have thrown in their lot with the daemons, ranging from simple peasants and/or laborers, to wizards, warriors, nobles, world leaders, and even the mighty Chaos Space Marines and Chaos Warriors .

The Chaos armies in Warhammer Fantasy and Warhammer 40000 share many similarities. Although the Eldar, Orks and Imperium definitely correspond to the Elves, Orcs and the Empire in the fantasy game, they do not always share the same religion or the same units (as the Chaos forces do with their daemons, Chaos Hounds, and Chaos Spawn). However, this similarity is not as close as it was in previous editions, where Fantasy Chaos armies and warbands could even include the occasional Chaos Space Marine.

The Horus Heresy

In the fictional history of the science fiction universe that the wargame Warhammer 40,000 is set in, the Horus Heresy took place after mankind's reconquest of the Galaxy, following the birth of Slaanesh. The Space Marines, the Imperium's elite and the mainstay of the conquering forces, became divided as some turned to Chaos and, led by the Warmaster Horus, turned on the Emperor's forces. This scenario mirrors the classic interpretation of the biblical tale of Satan's fall, with Horus corresponding to Satan, and the other, lesser, "Primarchs" corresponding to the fallen angels.

The Forces of Chaos

• Daemons (Used in both games)
• Hordes of Chaos (Mortals) (Warhammer Fantasy)
• Beasts of Chaos (Beasts) (Warhammer Fantasy)
• The Lost and the Damned (Warhammer 40,000)
• Chaos Space Marines (Warhammer 40,000)

The Gods of Chaos

Khorne is the Blood God. The fire that destroys, the Hound of War that exists for little more than endless slaughter. Every kill in war, every execution of a criminal, every senseless murder, is made in his name, whether those who commit them desire it (or even know) or not.

Nurgle is the God of Disease and Decay. The hungry earth, the Carrion Crow whose poxes and plagues are inflicted upon all. Nurgle's power waxes and wanes inconsistently as his "gifts" to the world(s) spread, are cured, or simply kill off everything in sight. Many of Nurgle's victims will turn to him to escape the torment of his diseases.

Slaanesh is the Dark Prince of Chaos. The beguiling water, the Great Serpent who lives for pleasure and the sensations of life and death. All those who live in the pursuit of pleasure risk falling under this god(-dess?)'s deceptive charms.

Tzeentch is the Changer of Ways. The inconstant air, the Great Eagle who pulls the strings of fate in whichever ways he pleases. The minions and followers of Tzeentch manipulate magic and psionic power in their rawest form, and often know the thoughts and fates of their foes even before they do.

Malal is the renegade God of Chaos, enemy to the other four. He represents absolute anarchy. He is, however, absent from the most recent editions of Warhammer games for copyright reasons. Some fans speculate that Malal is the force by which Be'Lakor, the Dark Master, the Greater Daemon of Chaos Undivided, introduced in the Storm of Chaos campaign, has gained power.