 When The Seer foresees the demise of The Source by the Halliwell sisters, he decides to break the covenant between good and evil unleashing the ultimate power, the forbidden The Hollow. He kills the Guardians of The Hollow and uses the dangerous lethal power to absorb the powers of Piper and Paige. Meanwhile, Phoebe has a premonition of Cole sacrificing himself to protect her against a ball of fire, and they try to find in the Book of Shadows means to vanquish the powerful demon and trap The Hollow again.