You can strike at enemy spellcasters who fail to cast defensively when you threaten them.
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.
Pathfinder RPG Core Rulebook. Copyright 2009, Paizo Publishing, LLC; Author: Jason Bulmahn, based on material by Jonathan Tweet, Monte Cook, and Skip Williams.