The Adversary by Julian May
The Pliocene Epoch's exiled races are caught in a violent struggle for ascendancy. The humans who escaped to the Galactic Milieu are now beset on all sides, as they seek a foothold in this turbulent land. Aiken Drum, now King, has many enemies, but the Firvulag seem set to move first to initiate their long-prophesied Nightfall War. And although recent confrontations have weakened Aiken, any sign of frailty will bring down his kingdom.
The powerful Elizabeth Orme supports King Aiken, and his enlightened despotism is preferable to Marc Remillard's cruel ambition. But these conflicts will be overshadowed if a time-gate is opened back to the 22nd century, something Marc can never permit. All will be decided at the Grand Tourney, where Tanu and humans will face the Firvulag in the last great contest of the exiled world.
Praise for the series: 'Julian May has woven a many-coloured tapestry of exotic adventure' Roger Zelazny, 'An engaging storyteller' Locus, 'Fast-paced storytelling that defies predictability; and a sympathetic and well-rendered cast' SFReviews.net