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Opportune Strike (Su)

Prerequisite: Magus 15, critical strike arcana

Benefit: Whenever the magus scores a hit with a melee weapon when making an attack of opportunity, he may cast a spell with a range of touch as an immediate action, and then make a touch attack with that spell against the target of the attack of opportunity as a free action. The magus can use this ability once per day.

Errata
Opportune Strike has some weird language, I think because you neglected to say that he must score a critical hit in the first phrase. Second, you can't perform a swift action when it isn't your turn.
Updated

Opportune Strike (Su): Whenever the magus scores a hit with a melee weapon when making an attack of opportunity, she may cast a spell with a range of touch as an immediate action, and then make a touch attack with that spell against the target of the attack of opportunity as a free action. The magus can use this ability once per day. The magus must be at least 15th level and possess the magus arcana (critical strike) before selecting this arcana.
Editor's Note: The update has been applied to the Opportune Strike magus arcana.