Amazon.com (ISBN 0061057797, Mass Market Paperback)

The demon Lailoken, as old as time, is tricked by Fire-lords and trapped in a human body. He becomes Merlinus, a wandering wise man expert in magic, destined to work for good among humans, opposed by the Furor (Woden). An encounter with the unicorn--a spirit similarly earthbound--brings him to Ygrane, queen of the Celts, and she sets him a task to find her king, a man seen in vision and fated to be her love-match. Merlinus-Lailoken seeks and finds him: Theodosius, a stable worker. But Ygrane has commanded the demon-wizard to bring her a king, so Merlinus sets to work making one.

This is an entrancing fantasy, drawing on everything from Norse myths to feng shui to build a magical, liminal Arthurian Britain.