In the Northern Wastes lies the Realm of Chaos. This is the home to the four Chaos gods: Tzeentch, changer of the ways; Khorne, the blood god; Nurgle, lord of decay; and Slaanesh, the dark prince. Each of these gods can be thought of as epitomizing rage, hope, despair and pleasure. In the Northern Wastes, Chaos seeps into the world, such great power causes the environment and inhabitants to become tainted and mutated by it. The inhabitants of the Wastes worship the Chaos gods, and thus the warriors that arise here are known as the hordes of Chaos. Generally, the northern tribes of men fight amongst themselves for their specific gods, but every so often, a champion arises and unites the tribes, spreading Chaos into the Old World, usually starting with the Empire to the south.
As the power of Chaos spreads, its army will continue to mutate and be "blessed" with new powers from the Chaos gods. Hence, units which were basic foot soldiers at the start may take on more powers and dark forms as the game progresses. Chaos offers great power with living armor and abilities not available to the men of the Empire. Chaos Armor is one of the most recognizable characteristics of the Chaos army. These suits of living armor offer unearthly protection against the enemy, often causing fear in those who stand in their way.
All this comes with a great price as the blessings from the gods often requires a painful existence of rotting and mutated flesh. But for Chaos, this is the ultimate gift.