Darkspell is a novel by Katharine Kerr published in 1987.

Rhodry is sent into exile by his brother Rhys, the Gwerbret of Aberwyn, and becomes a mercenary soldier called a “silver dagger.” Jill goes with him; they become involved in a dark dweomerman’s plot to steal the Great Stone of the West, a magical gemstone which guides the conscience of Deverry’s king.

Dave Langford reviewed Darkspell for White Dwarf #99, and stated that "if the language is fake-Celtic, why the Old English?

(To give a warm glow to D&D fans, I suppose.)

I lost heart at this point.