Charmed (1998): Season 4, Episode 15

4 - 15: Marry-Go-Round

As Phoebe prepares for her wedding day, she is completely unaware that the Source has taken over Cole's body, a plot that will likely ruin the big event. Meanwhile, when the Seer tells the Source that if he can marry Phoebe in the "dark way", it will insure that their future son will be the most powerful, evil being ever.