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In this variant, a character who has been the target of a spell sometimes finds that some of its magic rubs off on him or her permanently, leaving an echo of the original spell. A character who has alter self cast on her many times, for example, may develop the spell-like ability to alter her features into the specific form she's most familiar with. Beneficial spells can linger on a character like magic radiation, bestowing an advantage somehow related to the original spell. In contrast, some PCs have a homeopathic reaction to hostile spells; by suffering the effects of a spell, they develop a countermeasure to it.
Such spelltouched characters are a mystery to academic-minded spellcasters, who can't reliably duplicate the process by repeatedly casting the same spell on a subject. Magic interacts with each individual in a subtly different way.
Accordingly, the only way to become eligible to select a spelltouched feat is to have been exposed to (that is, targeted by or otherwise affected by) one of the spells associated with the feat. if the spell allows a save, you must have failed a saving throw against it at least once, whether intentionally or not. After meeting the prerequisite, you may select a spelltouched feat when your character would otherwise qualify for a feat. The exposure is the game-world explanation for your new power, and the feat choice is the trade-off that keeps the game balanced.
Metagame Analysis: Spelltouched Feats
Standard feats rarely give your character overtly magical powers. Instead, they represent outstanding natural ability or rigorous training. Some feats such as Spell Focus, Spell Penetration, and metamagic feats make your existing magic better in some respect, but they don't give you magical power you didn't already have.
Spelltouched feats break that rule wide open, giving characters abilities previously impossible without the aid of magic. But they're balanced against standard feats, so characters who choose them aren't necessarily more powerful than their nonspelltouched counterparts.
If you use this variant, you'll find that magic becomes incrementally more important in your game, and even characters who rely on their martial prowess, such as fighters, sometimes manifest a magical effect.
Because these feats are variants, they employ other game mechanics rarely seen in feats, such as a drawback that accompanies the feat or a limited number of uses per day or week.