THE DEEP

Origins

When the Serpent Queen chained the moon and called down it’s power to save her people, the stars began falling. Some faster than others and some slower, but those that reached the world before The Watcher took over the sky could not stop falling, and plunged into the depths below. They warped and mutated all they shone upon, twisting beasts into abominations, half-worldly and half of the moon. These Things roamed the surface world and began unmaking, ripping apart the elemental world and replacing it with that which is lunar (which accelerated the already quickly-collapsing Serpentine Empire).

Then The Watcher came and drove all these Things away, and they hid in the depths, so deep that the sunlight could not reach them and even deeper still, so that those following the sun could not reach them. And thus The Watcher stayed ever-vigilant, keeping back the stars above and the things below but they still lived and still stirred, and the world under the world grew more wretched and more horrible until it came to be known as THE DEEP.

Art of a star by one of my current players

Elves

While the snakes may have ruled the world and mastered the elements, the elves were clever and patient and hid in their hidden places and watched and waited. When the Serpent Queen failed to tame the moon, they watched and they waited and they learnt, and once the snakes were fully gone they emerged and began to experiment. They built a tower to the moon and learnt its secrets but then The Gods came and with them all their wars and strife. Some elves joined and some fled, but some were angry and vengeful for they believed they were on the precipice of greatness and as such delved to THE DEEP to continue their accursed research on the stars. 

Ignorant of their workings, the world above carried on for a time, until these deep-elves believed themselves ready, masters of the moon and the stars and the Things of the world below. The deep-elves tunneled upwards and broke ground ready to claim the world as theirs but the world above had changed too, and they found themselves met by the elves above, guided by their new Seer goddess and with the new practice of "wizardry", it was a mighty fight indeed. They fought in the rivers and the valleys, in the woods and the mountains, and that tunnel upwards from THE DEEP has turned a corner of the great elven forest into a Cloakwood, and the fight continues to this day. Many a surface-elf devotes their life to mastering the arts of blade and spell to redeem or slay their fallen brethren, and hold back these Things from the rest of the unwary world.

Art of a deep elf drawn by one of my past players

Dwarves 

The dwarves live in their holds and dig deep, but their holds are in mountains so often they do not dig so deep after all. But the ones that do, both the great and mighty cities that go further and further below as well as the weak cities that were never built into a mighty mountain in the first place, they will always inevitably dig into THE DEEP. Of course they could be content with what they have and stop digging but that is not their way, instead they prepare and build traps and turn their holds into impenetrable fortresses to be attacked from two fronts, one from the peoples above and another from THE DEEP below. And often this works for they only scrape at the top layers of THE DEEP far from the fallen stars, save for one notable exception.

It is unknown whether the dwarven city-fortress of Deephold was named such because it was planned to be dug deeper than any other before it, because it was built to defend against THE DEEP, or if the name was retroactively given after the connection was made, but either way it was dug as a home for the King Under the Mountain, ruler of the Western dwarves. Deephold thrived while below it an evil was bubbling, and THE DEEP was breached deeper than ever before. A truly horrible Thing emerged, one which reshaped the world around it with naught but a thought as easily as it could reshape the moon from where it hailed. The king fell in battle and so too did his armies, and the Western dwarves shattered into three disunified kingdoms. The Thing fell asleep as it was to do, yet still dreamed and reshaped fallen Deephold while an incursion from THE DEEP spilled forth.

This incursion was weaker and less organized than that of the deep-elves and as such the dwarves beat it back easily, for all their cities are prepared for such incursions, and the beasts were slain all the way up to the gates of the fallen city, which were sealed shut with powerful magics and a town built atop, to ensure all would be well. This lasted for hundreds of years until a descendent of that King Under the Mountain thought himself greater than his forefathers and unsealed the city. Thus the Thing's dream escaped and spread across the mountains to envelop the other dwarven kingdoms, but the heir infiltrated the dream and convinced the sleeping Thing to dream itself out of existence, and from then the dwarven armies could easily push the beasts all the way back to THE DEEP from where they came and reclaim Deephold, to begin the process of uniting the dwarves once again.

My banner for the campaign in which Deephold was reclaimed (they all had darkvision)