Realm of Chaos: Slaves to Darkness (1988), p123 — 466-475 Horns

The mutant's head grows horns, and it gains a gore attack if it does not 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 it as a servitor of its Chaos lord.

Chaos Armour will change to accommodate this attribute, but other headgear must be discarded.