You can zap enemies who interrupt your spellcasting, using residual magic.
Benefit: If your spellcasting is interrupted by a melee attack and your spell is lost, you may make a touch attack (considered armed) as an attack of opportunity against the attacker. This attack deals 1d6 hp force damage/level of the spell lost.
Section 15: Copyright Notice – Advanced Feats: Visions of the Oracle
Advanced Feats: Visions of the Oracle. Author: Sigfried Trent Copyright 2011, Open Design LLC, www.koboldquarterly.com. All rights reserved.