Sham's Superior The Enemy Within
A Chaos Mutant could be defined as a organism (be it plant, animal or humanoid) metamorphosed or mentally altered by the warping influence of Chaos, thus including the Beastmen, half-human and half-beast, the Skaven or ratmen, as well as numerous other, possibly nameless, creatures. However, ‘Chaos Mutant’ commonly refers to a human somehow changed by the touch of a mutation.
In the lands of civilized men, mutations are less frequent than in the North, but still appear quite often. In places such as Sylvania or Mousillon where Dark Magic is stronger, mutations are even common. These mutants are often killed at birth or hidden from the spying eyes of the Order of Sigmar. If they survive the hunt, they have to live a hidden life and flee into remote places where they band together creating groups of mutants ranging from people with small marks to the Beastmen and much worse. When the power of Chaos rises, they might take up arms and break free from their hidden existence to execute their revenge upon a civilization from which they were banished.