Daggerspell

Even as a young girl, Jill was a favorite of the magical, mysterious Wildfolk, who appeared to her from their invisible realm. Little did she know her extraordinary friends represented but a glimpse of a forgotten past and a fateful future. Four hundred years--and many lifetimes--ago, one selfish young lord caused the death of two innocent lovers. Then and there he vowed never to rest until he'd righted that wrong--and laid the foundation for the lives of Jill and all those whom she would hold dear: her father, the mercenary soldier Cullvn; the exiled berserker Rhodry Maelwaedd; and the ancient and powerful herbman Nevyn, all hound in a struggle against darkness ... and a quest to fulfill the destinies determined centuries ago.

This is #1 in Deverry Series.