Karmic Strike( Complete Warrior, p. 102)
You have learned to strike when your opponent is most vulnerable - the same instant your opponent strikes you.
You can make an attack of opportunity against an opponent that hits you in melee. On you action you choose to take a -4 penalty to your Armor Class in exchange for the ability to make an attack of opportunity against any creature that makes a successful melee attack, or melee touch attack against you. The opponent that hits you must be in your threatened area, and this feat does not grant you more attacks of opportunity than you are normally allowed in a round. You specify that you are activating this feat, and the change to Armor Class and your ability to make these special attacks of opportunity last until your next turn