The Charmed Ones are swept into the pastUnbind A Bond, To: The sisters cast this spell to undo the pact that Nicholas made with their mother. By casting it they inadvertently sent themselves into the past.

The bond which was not to be done,
Give us the power to see it undone,
And turn back time to whence it was begun

Underworld, The: See: Demon Dimension.

Unholy Union: A marriage to evil. This ceremony, if completed between a Witch and a Warlock, is capable of turning a Witch to evil. When the Warlock Zion bound Prue Halliwell to himself he was able to make her, her sisters, and their Book Of Shadows evil.
Zion married Prue while she was in magick-induced slumber, and in the space of half a day her transformation was complete. Becoming Warlocks Prue and her sisters all gained extra magickal abilities. All could blink, while Phoebe received the powers of telekinesis and transformation, and Piper transformation and cryokinesis. The Book Of Shadows similarly changed with new spells appearing, and evil able to touch it and wield its magicks.
The union was broken with the death of Zion, and the evil undone. The Halliwell sisters became good Witches once again, and the Book Of Shadows returned to normal.
An unholy Union is similar to Handfasting, and must be conducted by a dark High Priestess. The participant’s hands are bound as a symbol of unity, the Priestess consecrating the ceremony with these words:

In the beginning we were damned, and through damnation we found freedom, power and purpose. As I unite you today, I remind you of those gifts, and in your union, may these gifts increase, so your powers may grow in the service of evil. So be it.

Warlock Prue    Prue slumbers while she is turned to evil