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Persistent Spell (Metamagic)
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. 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.