The Saga of Pliocene Exile by Julian May.
Volume I The Many-Colored Land: When a one-way time tunnel to Earth's distant past, specifically six million B.C., was discovered by folks on the Galactic Milieu, every misfit for light-years around hurried to pass through it. Each sought his own brand of happiness. But none could have guessed what awaited them. Not even in a million years...
Volume II The Golden Torc: By A.D. 2110 nearly 100,000 humans had fled the civilized strictures of the Galactic Milieu for the freedom they thought existed at the end of the one-way time tunnel to Earth, six million B.C. But all of them had fallen into the hands of the Tanu, a humanoid race who'd fled their own galaxy to avoid punishment for their barbarous ways. And now the humans had made the Tanu stronger than the Firvulag, their degenerate brethren and ritual antagonists. Soon the Tanu would reign supreme. Or so they thought . . . .
Volume III The Nonborn King: On Earth, six million B.C., two species of alien ruled, the graceful humanoid Tanu and their twisted brethren, the Firvulag. Then men from twenty-second century Earth arrived through a one-way time tunnel -- and soon the aliens were locked in a battle to the death, for the humans had upset the precarious balance of power that existed between them. But when the tides of combat had receded, no one group held firm control, though Aiken Drum, man of no woman born, had declared himself the Nonborn King . . . .
Volume IV The Adversary: The fourth and final volume of The Saga of Pliocene Exile. Until the arrival of Aiken Drum, the 100,000 humans who had fled backward in time to Pliocene exile on Earth knew little but slavery to the Tanu -- the humanoid aliens who came from another galaxy. But King Aiken's rule is precarious, for the Tanu's twisted bretheren are secretly maneuvering to bring about his downfall. Worse -- Aiken is about to confront a man of incredibly powerful Talents who nearly overthrew a galactic rule. He is Marc Remillard. Call him . . . The Adversary.
Books are in fair to good condition with some cover damage.