Spellbreaker

Combat

You can strike at enemy spellcasters who fail to cast defensively when you threaten them.

Prerequisite: Disruptive, 10th-level fighter.

Benefit: Enemies in your threatened area that fail their checks to cast spells defensively provoke attacks of opportunity from you.

Normal: Enemies that fail to cast spells defensively do not provoke attacks of opportunity.

Source: Pathfinder Core Rulebook