The Histories of King Kelson #3: The Quest for Saint Camber by Katherine Kurtz

Time after time, the legendary Camber–one-time Saint of the half-magic race of Deryni–had intervened in the affairs of young Kelson, King of Gwynedd. Yet Camber was known to have died two hundred years before!

Now Kelson set forth with his blood brother Dhugal to search out relics of this mysterious patron of his, leaving his uncle Nigel to rule in his absence.

Then word came that Kelson and Dhugal had fallen from a crumbling cliff during a storm. No trace of their bodies could be found int he raging waters below. The king was dead and Nigel must become king.

But to Conall, son of Nigel, the temptation of the throne was too great. In rage, he struck down his father with secretly gained Deryni power. Now he would be king!

But Conall had overlooked Saint Camber…