Hecate

Hecate was queen of the underworld. She appeared every 2000 years and would wed a fully human man, by doing so she could fornicate and have a child which on the outside would look good but be internally and mentally. A poingard would be needed to kill her. In Season 1 (The Wedding From Hell ), Hecate planned to wed a man named Elliot, leaving his true fiance Allison quite distraught.

For a brief time Phoebe thought that perhaps Piper was hecate and so confessed to Piper that when she found the poignard she would have to kill her.