Counterstrike (3.5e Feat)
From D&D Wiki
Through tactical training you have mastered the deadly precision of allowing enemies to enter close proximity and use your retaliatory strikes as a counter during triggering attacks.
Prerequisite: Combat_Focus_(3.5e_Feat), Improved_Combat_Focus_(3.5e_Feat), OGC:Defensive_Stance_(3.5e_Feat)
Benefit: If you choose to hold your turn and are readied in a defensive stance, you can use your BAB bonus or Wisdom Modifier as a swift action upon being attacked as a chance to counter enemy attacks. You gain a bonus on attack rolls and damage rolls equal to the number of enemies attacking you specifically, and must make an attack check that beats their Flat-footed AC to retaliate unimpeded. You immediately lose all benefits when a counterstrike attack has failed, resulting in your turn ending for the round.
Normal: Whenever you use Counterstrike, you gain a +2 bonus on each extra weapon damage dice roll this feat grants (+10 maximum). This bonus damage is multiplied on a critical hit.
Special: Whenever you make a melee attack against a flanked foe or against a foe denied their Dexterity bonus, you deal an extra 1d6 points of damage, and your weapon's threat range is doubled (as if under the effect of Keen). The increased threat range granted doesn't stack with any other effect that increases your weapon's threat range. Creatures immune to extra damage from sneak attacks are immune to the extra damage dealt by your attacks. NOTE:Counterstrike has a cooldown, meaning once used, the ability can't be used again for 1d4 rounds.