The Spellthief's Steal Spell Effect ability

Steal Spell Effect (Su): Beginning at 2nd level, a spellthief can siphon an active spell effect from another creature. A spellthief who hits an opponent with a sneak attack can choose to forgo dealing 1d6 points of sneak attack damage and instead gain the effect of a single spell affecting the target. If the target is willing, a spellthief can steal a spell effect with a touch as a standard action.

The spellthief can choose which spell effect to steal; otherwise, the DM determines the stolen spell effect randomly. If a spellthief tries to steal a spell effect that isn't present, the stolen spell effect is determined randomly from among those currently in effect on the target. A spellthief can't steal a spell effect if its caster level exceeds his class level + his Charisma modifier.

Upon stealing a spell effect, a spellthief gains the stolen effect (and the original creature loses that effect) for 1 minute per class level (or until the spell's duration expires, whichever comes first). If the spell effect's duration hasn't expired by this time, the spell effect returns to the creature that originally benefited from it.

A spellthief can steal the effect of a spell only if the spell could be cast on him by the original caster. For example, a spellthief couldn't gain the effect of an animal growth spell (unless the spellthief is of the animal type) or the effect of a shield spell (since that spell's range is personal). If a spellthief tries to steal the effect of a spell not allowed to him, the effect is still suppressed on the original target of the spell for 1 minute per spellthief class level.

This ability does not work on spell effects that are immune to dispel magic (such as bestow curse).