Julian May's Saga of the Exiles tells the story of human malcontents who leave the Galactic Milieu, a Federation of six allied psychic races including Humanity, and travel via a Portal To The Past to the Pliocene Era of Earth's history 6,000,000 years ago. The past is not quite what anyone expected.The time-travelers arrive to discover the Pliocene is already inhabited by a dimorphic race of aliens ('exotics'). The exotics, who have fled their home galaxy because of religious persecution, are marooned on Pliocene Earth as well. The exotics have difficulty reproducing on Earth, and so have enslaved many of the humans in an effort to overcome this problem, interbreeding with the more robust humans and influencing the legends of Celtic Mythology and The Fair Folk.The books making up the tetralogy are;
Big Dam Plot - Felice destroys the natural structure at Gibraltar that keeps the Atlantic from filling the Mediterranean Sea, thus wiping out the Tanu capital city in a Giant Wall of Watery Doom.
Bread, Eggs, Milk, Squick - Felice's dagger is useful for "picking teeth, cutting sandwiches, pricking out transponders from rustled cattle, and putting out the lights of casual assaulters."
Brought Down to Normal - Elizabeth after her accident in the Milieu. Her power is then restored by the trauma of passing through the time-gate.
Butt Monkey - Tony Wayland, oh so much. This guy just can't catch a break Until the very end. Of course, this is to be expected given that he is a not so subtle nod to the mythical Wayland Smith.
The Fair Folk - The Tanu and Firvulag, who match well to the Seelie and Unseelie Courts respectively, the Tanu representing the tall and beautiful archetype, while the Firvulag the darker, more goblin-like side. Most of the named exotics correspond directly to members of the Tuatha de Danaan and the Fir Bolg or Fomor — Nodonn/Nuada, Lugonn/Lugh, Pallol/Balor, et cetera.
Fantasy Gun Control - the types of weaponry permitted through into Exile are strictly controlled, and the Tanu confiscate it all anyway. The Tanu and Firvulag don't use firearms, except for some of their ritual weapons.
Older Than They Look - Many of the the Ocala rebels have undergone rejuvenation, so they look thirty or forty but are really a few decades older. Owen Blanchard, the oldest of the Rebels, is about a hundred and forty and looks to be a normal old man. Marc is also self-rejuvenating; he looks fairly young but is actually about seventy at the time of the series.
Psychic Powers - The Tanu are organised into five psychic guilds: each guild has a guild leader and its own heraldic colours, which allow others to instantly recognise the person's primary metapsychic power. Coercive (Blue), Creative (Green), Psychokinetic (Rose-gold), Farsensing (Purple) and Redactive (Red).
Shapeshifting - Many characters are able to shapeshift through use of the Creative Metafunction. Aiken Drum turns into a series of different animals reflecting his personal growth and increased metapsychic strength. Marc turns his son Hagen into a fish for a while as a punishment and In the later books Felice can turn into a raven. Also see Voluntary Shapeshifting below.
Stable Time Loop - Marc Remillard and Elizabeth Orme travel to the Duat galaxy to help them with their psychic development. Afterwards Marc returns alone to the Milky Way and attempts to raise the native species to operant metafunction. His first success is with the Lylmik, who are the ones who create the Galactic Milieu, against which the Metapsychic Rebellion take place, causing Marc to go back to the Pliocene where he meets Elizabeth in the first place...