The Serpent and the Rose — Kathleen Bryan

FORBIDDEN MAGIC, FORBIDDEN LOVE

In the Kingdom of Lys, where magic is worked in the form of Jeweled glass, the Knights of the Rose and the Ladies of the Isle guard the world against an ancient enemy. Long ago, the Serpent was imprisoned by the Young God, and the Orders still keep the secret of that prison a thousand years later. But now a traitor king is conspiring to destroy the heart and soul of his kingdom. Only Averil, heir to the duchy of Quitaine, and Gereint, the fatherless son of a peasant farmer, have the power to stand against him.