The Isle of Glass — Judith Tarr

Alfred of St. Ruan's Abbey in Ynys Witrin was content to live, as he had for decades, as a quiet monk among his brethren, who did not remark on his fair face, his ageless body, his ability to heal...no one at St. Ruan's would think of asking Brother Alf if he were of the Elven-kind.

But the world intervened. One night a grievously injured ambassador from the Court of Gwydion, the Elvenking, came knocking on St. Ruan's gate, drawn by Alf's powers. The knight Alun had been on a mission to avert a war between young Kilhwch of Gwynedd and Richard the Lionheart, a mission that must be completed within days. Alf, commanded by his Abbot, reluctantly accepts Alun's charge to go to King Richard with the news of treachery--and with that begins his journey into the center of royal conflicts and religious controversy. For the world beyond St. Ruan's gate is not willing to accept one of the Fair Folk as a priest of God....