Your spells carry an embedded curse that manifests when they strike true.
Benefit: When you confirm a critical hit with a spell or spell-like ability, you may cast bestow curse or major curse on that target as an immediate action. This works even with ranged spells. You must have bestow curse or major curse prepared or otherwise available to cast, and using this ability casts the corresponding spell.
Pathfinder Roleplaying Game Ultimate Magic. © 2011, Paizo Publishing, LLC; Authors: Jason Bulmahn, Tim Hitchcock, Colin McComb, Rob McCreary, Jason Nelson, Stephen Radney-MacFarland, Sean K Reynolds, Owen K.C. Stephens, and Russ Taylor.