Chaos

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 warriors, nobles, world leaders, and even the mighty Chaos Space Marines.

The Horus Heresy
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.

The Forces of Chaos

 * Daemons
 * The Lost and the Damned
 * Chaos Space Marines

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.