Knucklehead (3.5e Feat)
From D&D Wiki
 Knucklehead [General, Maneuver]
Prerequisite: Constitution 13
Benefit: In a fight, you may exchange one of your attacks that round for a headbutt attack. The headbutt attack is regarded as an unarmed strike made with the head. Headbutting with a helmet on deals 1d6 base damage and the damage is lethal. You may make a headbutt even when being grappled, without taking the usual -4 penalty to attack. You add 1½ your Strength modifier to the damage of any headbutt.
Main Page → 3.5e Homebrew → Character Options → Feats → Maneuvers