In the original Charmed Season 1, Episode 7 ("The Fourth Sister"), a troubled witch named Aviva is manipulated by the evil sorceress Kali to steal the Halliwell sisters' powers. The episode, featuring the first appearance of fireballs, concludes with the sisters saving Aviva and defeating Kali. Detailed plot and trivia for this episode can be found at Charmed Wiki . The Fourth Sister - Charmed Wiki