The House of Gaian — Anne Bishop

It began as a witch-hunt. But the Master Inquisitor's plans to eliminate all traces of female power in the world have expanded to crushing the Sylvalan barons who oppose him--and to destroying the wellspring of magic in the Mother's Hills.

Faced with this evil, humans, witches, and the Fae become uneasy allies. But even together, they aren't strong enough to stand against the armies the Inquisitors are gathering. So they look for help from their last possible ally. The House of Gaian. The reclusive witches who rule the Mother's Hills. The witches powerful enough to create a world--or to destroy one...

And the witches' long-held creed "Do no harm" is about to give way to a more important one: Survive.