Mark of Chaos

A Mark of Chaos is a sign of favour from one of the Ruinous Powers, often bestowed upon a loyal subject. It is a brand connecting the person to his foul deity, and an outward sign of his devotion. Marks of Chaos can come in many different shapes, styles, or designs, but are clearly associated with their Ruinous Power and often the actual symbol of that god. Here are just a few examples. Followers of the same Ruinous Power have an uncanny intuition, allowing them to sense when someone bearing their deity’s Mark of Chaos is nearby. Bearers of a Mark of Chaos usually go to great lengths to hide it from non-believers, as recognition of such a sign is a death sentence, sure to draw the attention of Witch Hunters, angry mobs, or worse.
 * A Mark of Tzeentch may manifest as a pulsing blue Chaos Star, a raven tattoo that appears to be flying across the subject’s skin, or a sinuous, writhing flame emblazoned on the subject’s back.
 * A Mark of Nurgle may be a distended boil oozing foetid pus, an open sore weeping a stream of maggots, or a pox-marked patch of vaguely green-tinted skin that constantly sheds and peels.
 * A Mark of Khorne may change the marked person’s eyes to deep, blood red, it may manifest as a dark red battle scar still wet with crimson blood, or a skull-shaped knob or deformity on the marked one’s flesh.
 * A Mark of Slaanesh may be a subtle, intoxicating scent of musk lingering to the person, a slightly dreamy, purplish tinge to the marked one’s lips, eyes, and hair, or the growth of additional nipples.

Source

 * Warhammer Fantasy RPG 3rd ED -- Book of Change
 * pg. 20