Prerequisites
Base attack bonus +12, proficiency with selected weapon, Weapon Focus and Greater Weapon Focus with weapon group containing the selected weapon, Weapon Specialization with selected weapon.
Benefit
Choose one type of weapon for which you have already selected the Weapon Specialization feat. The damage you deal with the chosen weapon is increased by an additional three steps as indicated on Table: Damage Progression.
For example, applying this feat to longsword would increase the damage from 1d8 to 2d8 with Weapon Specialization and then from 2d8 to 4d6 with this feat.
Special
You can gain Greater Weapon Specialization multiple times. Its effects do not stack. Each time you take the feat, it applies to a new type of weapon.