Deryni Rising by Katherine Kurtz

The classic novel that introduced the Deryni and launched Katherine Kurtz's career.
In the land of Gwynedd, the Haldanes have long ruled and have long kept a dangerous secret: there are those of their blood who possess the magical powers of the Deryni. To be Deryni in a land ruled by the all-powerful Church is to be branded an outcast.
But now, young Prince Kelson is about to assume the throne after the mysterious death of his father. He must be told of his magical heritage. For his legacy is being challenged by a woman who does not hesitate to lay full claim to her Deryni powers. And to face her in magical combat, Kelson must learn a lifetime's worth of magic in a few short days.
If he loses, he dies as his father did. And if he wins, he is King-but all the world will know that he is also Deryni.