Source: Warhammer Fantasy: 6th Edition
The creatures of Chaos are not merely beasts of savage mind and brute strength, though even if they were they would be threat enough. What makes them even more dangerous is their place in the fold of the Dark Powers.
Some Beastmen are granted gifts by the god they take as their own patron, while others simply revel in the anarchy of Chaos in its purest form. Whichever is the case, with the favour given them by their gods, the Beasts of Chaos become ten times as foul and ten times as dangerous.
Characters, units of Chaos Warriors, Chaos Knights, Chaos Chariots, up to one unit of Bestigors, and Tuskgor Chariots are assumed to have the Mark of Chaos Undivided. This costs no points. They can replace the Mark of Chaos Undivided with the Mark of a specific Chaos god at an additional cost. Each of the five Marks bestows benefits on the character, unit or chariot bearing it. The Mark affects all models in the unit and only characters bearing the same Mark can join the unit/ride in the chariot.
Wizards with the Mark of a specific god also gain access to their god's spell lore (which they must use).
A character or unit may only be given a single Mark.