The Tower and the Emerald by Moyra Caldecott

Set in a legendary Britain of long ago, The Tower and the Emerald is an epic fantasy of good and evil, magic and mystery.

Viviane, a beautiful Celtic princess, unwittingly unravels the spell that binds the spirit of the evil Idoc within a circle of tall stones. Once released, the sorcerer-priest uses his powers to deliver vengeance upon those responsible for his original enslavement, including the Princess Viviane.

With Idoc in possession of the body of Prince Caradawc, her betrothed, Viviane can no longer judge safely between friend and foe, between this life and previous incarnations. Yet to rescue Caradawc from his nightmare, she must risk everything to reach the dreaded dark tower where Idoc waits, and to ensure victory over evil, seek out Lucifer's Emerald.