Combat Fury

 
Prerequisite(s): Rage 2+/day, Base Attack Bonus 3+
 
Benefit: By spending an additional Rage per day while you are already enraged, you can enter a combat frenzy that enables you to make one additional attack per round at your highest attack bonus for the duration of your rage. This bonus attack and all other attacks made until your next action suffer a -2 penalty to your attack roll, and you must take the full attack action in order to make this extra attack. In addition, when using this ability you increase your AC penalty to -4 during any round that you make this extra attack.
 
Source: Mongoose Ultimate Feats