S04E13: Charmed and Dangerous

The Source breaks an age-old agreement between good and evil by stealing The Hollow, an ancient vapor which he uses to absorb Piper & Paige's powers. Meanwhile, Phoebe must allow Cole to help them, despite her having a premonition involving Cole sacrificing his life to save her from evil, but there may be more to the premonition that Phoebe just can't see.