The Golden Torc by Julian May
A small group journeyed through a time-gate into Europe's prehistoric past. Yet this supposedly unspoilt sanctuary holds two alien races locked in combat. In a world where the human-like Tanu have the upper hand, Elizabeth Orme soon encounters trouble. When they find she possesses rare mind powers, they want her for their own. She won't be used as a pawn in a Tanu versus Firvulag war, but Aiken Drum can't wait to get involved.
Aiken discovers the Tanu's mind-enhancing torcs have given him his own powerful abilities. And it's not long before he devises a plan to challenge the Tanu's leader - for rule of the Many-Coloured Land itself. But another faction seeks the slaughter of all humans, and he stands in their path.
Praise for the series: 'Enchanting and engrossing ... I was captivated' Fritz Leiber, 'Julian May has woven a many-coloured tapestry of exotic adventure' Roger Zelazny, 'An amazing journey from the distant future to the distant past ... high adventure' SFReviews.net