You’ve learned how to use your cranium to smash your foes.
Benefit: You gain a natural attack that deals 1d3 points of damage (1d2 for Small characters, 1d4 for Large characters). If you have an unarmed attack that deals more damage, you may choose to deal that damage instead when you use this natural attack. If you damage a foe with this natural weapon, you take 1/4 of the damage you inflict (if you have damage reduction, it does not reduce the share of the damage you suffer). You can use this natural attack without penalty even if your hands or feet are bound, or if you are in a grapple (you do not have to make a grapple check to use this natural attack when you are involved in a grapple).
Adventurer’s Handbook: Genius Guide Volume 1