The mutant's head grows horns and it gains a gore attack, if it doesn't already have one (A +1). The mutant also gains one fear point. As the horns develop, they often twist together into a likeness of the symbol of the mutant's Power, marking the creature as a servitor of its Chaos lord.
Chaos Armour will change to accomodate this attribute, but other headgear must be discarded.