Charmed (1998): Season 6, Episode 16

6 - 16: The Courtship of Wyatt's Father

Gideon makes a deal with the Darklighters to get rid of Leo, but they accidentally send Leo and Piper to the Ghostly plane, leaving Phoebe and Paige to believe that they are both dead. Meanwhile, Chris begins to disappear and reveals that he must be conceived before the night is over or he will vanish forever.