High Deryni by Katherine Kurtz

The enemy King of Torenth, whose daughter murdered Kelson's father and was in turned destroyed by Kelson, is moving against Gwenydd. He first sets out to destroy Morgan's lands and people and finally offers Kelson a devil's bargain. He and three of his people will meet Kelson and three others in an arcane duel, winner take all - since the four losers will be dead.

Murder, betrayal, and all the rest. And, again, previously hidden allies surface and save the day. The ending of this one was a surprise to me, Kurtz sneaked this one in, although, after revelations in the previous book, I suppose I shouldn't have been quite so surprised.

Good read. I enjoyed these years ago - and reread them a number of times. And I enjoyed them again.