As the Old Galactic Empire fell apart, during what we call the late Pleistocene, specifically about 112k YA during the Sangamon/Eemian Interglaciation, a starship made its way out into the frontiers of the Galaxy, far beyond any working jump-gate, and took orbit around our small planet. The starship was crewed by refugees, or revolutionaries, or criminals, depending upon your perspective, but none disagree that they were no longer safe in known Imperial Space.
They set up operations on a small continent, tectonically unstable, but rich in resources. The operation was to be brief - a rest stop to re-supply the Mother Ship – but something went Very Wrong in orbit, and the landing party found itself stranded in this primitive
paradise. Or, alternately, depending on the source, the Mother Ship recalled them and when they refused to go, they were abandoned. Or, they were simply abandoned.
The true names of the original landing party are lost, but their deeds have been associated (by humans who survived them) with names like Atlas, Votan, Zeus, Asmodeus, Marduk, Indra, Thoth and a host of others that populate world mythology. They possessed star-age technology, but little infrastructure to support it. For that they needed labor, and began modifying the local fauna, including a species of hominids peculiar to this continent, to serve as beasts of burden.
Among their wondrous artifacts, though, was a “wish machine”, a matter/energy converter that can be controlled by thought; the pinnacle of Titan technology, and almost certainly stolen by this particular group.
Sheltered by their isolation, and cushioned by their stolen technology, their passions ran unchecked by hardship for centuries, and the whole planet became their footstool. Their civilization was a long, destructive party that raged for thousands of years. But their slaves kept learning, and became more and more independent, running a shadow society around and in spite of the quarreling Gods, who took it all for granted until it was far too late.
Civil war, a slave insurrection, and tectonic instability finally caught up to them, and they were forced to abandon their civilization. Many abandoned the planet altogether, but a few remained: The Titans, the Aesir, the gods of Egypt and Mesopotamia and Greece. But they were too few to call a civilization, and the world, as it were, had passed to humans.
The Sidhe
The Titans were not the only refugees from the collapse of the Old Imperium. Previous to their arrival, perhaps as early as the Eocine, a faction of the Sidhe, a race of similar technical achievement, had planted a small refugee camp upon the planet. When adapted to terrestrial conditions, these creatures were better known as Elves and Fairies, and a host of other names.
The Sidhe were not as activist as the Titans, preferring to blend into the ecosystem, rather than rework it to their convenience. They had a mutual policy with the Titans of ignoring one another, the way old enemies will ignore each other at a polite party.
Even after the Titans had dispersed, Elves would visit Atlantis, but never dwell there.
Their conflict with the Titans came to a head over slavery. Where the elves only interacted with the natives to discourage them from trespassing into their realm, the Titans actively subjugated them, and the Elves eventually (Elves do nothing in haste) grew disgusted by this, and worked to subvert this. They taught a few humans “magic”, and this fueled the slave rebellion that figured so prominently in the Titan’s downfall.
But as the Titans fell, their Wish Machine began to lose its power, from which the Sidhe were drawing liberally as well. Thus, the spirits of the forest passed quietly but inevitably into myth.