The Third Magic — Welwyn Wilton Katz

Mistaken for one of her own ancestors, Morgan Lefevre is summoned through time to the alien world of Nwm. With only the boy Arddu as companion, she is caught between the opposing cruelties of the Circle and the Line, the two Magics of Nwm that have fought one another over the centuries and across many worlds. When Morgan and Arddu gain possession of an ancient weapon of the mysterious Third Magic, Earth is drawn into the struggle as well, and the reader comes to understand the King Arthur story in a whole new way. Times and worlds merge as Morgan is brought face to face with her own destiny.