The Book of the Cauldron — Diana L. Paxson

The Romans came, conquered, and departed, leaving behind them a strange new religion called Christianity. The princes of Britannia have driven back the invading Saxons and dream of a return to bygone days, while others on the island invoke the magic of a time older still. For the revered ancient goddesses Eve on in the hearts and memories of the proud royal women of the old British tribes. And now, King Artor -- wounded in body and spirit -- proclaims that the sword and the spear must be put to rest, and summons his mother Igierne, Lady of the Lake, to use the powers of the Cauldron to heal a troubled, wartorn land. But old jealousies and new power struggles threaten disaster, as Artor's sister Morgause hides a deadly secretand a darker magic. It is Guendivar the queen who holds the key to their survival, if she can master her own power in time.

For a darkness is emanating from high places -- and it threatens to devastate a realm.