Counterstrike, Variant (3.5e Feat)
From D&D Wiki
Characters with this Feat may react to an enemy's attack, striking back at them on instinct. If an enemy in melee range attacks this character, that character may roll a Concentration check to focus all of their speed and power into a quick counterstrike. The DC for this check is equal to 10 + Enemy Level + Enemy's Dexterity modifier. This is considered an Attack of Opportunity, and is subject to the same limitations per round.
Prerequisite: Dexterity 14, Combat Reflexes
Benefit: Make an Attack of Opportunity against an enemy attack