Dark Castle, White Horse — Tanith Lee

IN DARKNESS SHE WAKES--
a beautiful young woman, forever imprisoned in the Castle of Dark. Guarded and bespelled by two old hags, can she master the secret magic to summon the champion who will set her free?

INTO DARKNESS HE RIDES--
a handsome prince in an unknown realm. Who has conjured him here, and for what desperate deeds? Hailed as the Looked-for-Deliverer, with a changeling horse for companion, only he can challenge all magic's perils--from the Dragon of Brass to the Mad Witch to the kingdom-destroying horror, the evil, enigmatic Nulgrave!