Advanced Martial Arts Feat

Advanced Martial Arts
You are a master of unarmed combat due to your advanced training.
Prerequisite: Martial Arts, Improved Martial Arts, base attack bonus +8.
Benefit: A Medium-size character deals 3d4 damage with an unarmed strike. Strength modifiers apply. Also, you threaten a critical hit on a natural 18, 19, or 20 when making an unarmed attack.
Small characters deal 3d3 damage and large characters deal 3d6 damage with an unarmed strike and this feat.