You can modify a spell to become more tenacious when its targets resist its effect.
Benefit: Whenever a creature targeted by a persistent spell or within its area succeeds on its saving throw against the spell, it must make another saving throw against the effect. If a creature fails this second saving throw, it suffers the full effects of the spell, as if it had failed its first saving throw.
Level Increase: +2 (a persistent spell uses up a spell slot two levels higher than the spell’s actual level.)
Spells that do not require a saving throw to resist or lessen the spell’s effect do not benefit from this feat.
Advanced Player’s Guide. Copyright 2010, Paizo Publishing, LLC; Author: Jason Bulmahn.