Hecate is the (presumably) self-proclaimed "Queen of the Underworld" who comes to earth every 200 years to find an innocent man from a well-born family and put him under her spell so she can create a demonic spawn. She was the first upper-level demon encountered by the Charmed Ones.
Hecate must marry the innocent man in a sanctified wedding before impregnating herself. That way, her child would look human on the outside, but internally and mentally, he would be pure demon.
Her spell can only be broken by a declaration of true love and a sealed kiss. Just like the Noxon demons she cannot be vanquished easily. However, she can be cursed into a special 14th century Italian dagger called a poignard. The sisters run after Hecate and Elliot and find them in Elliot's room, just as Jade is morphing into her demon form and about to force Elliot to impregnate her. The bridesmaids, also in demon form, rush in after them. Prue cannot find the poinguard but it turns out to be on the floor next to Elliot. Elliot raises the dagger, and Prue telekinetically draws Jade and the bridesmaids into it, banishing them to the Underworld.