Undersong

When you cast this spell, a familiar and soothing song wells up in your mind. The song does not distract you from any task at hand—on the contrary, by humming along to the tune you can focus your mind with ease. For the duration, whenever you make a Constitution saving throw to maintain your concentration on a spell, you can choose to make a Charisma (Performance) check, substituting the result of that roll for the result of a Constitution save. 

“Quite useful—if ye can hold a tune.”    —Elminster