Atlanteans

Atlanteans were an subset of ancient human. Highly advanced their empire originally flourished on the island of Atlantis in the North Atlantic Ocean. They angered their gods who destroyed their paradise and sent Atlantis to the bottom of the ocean.
Not much is known for sure about the Atlanteans, and it was word of mouth that initially spread the legend of these people. The Egyptians followed by the Greeks, and then the Romans took up & recorded the legend of Atlantis & it's people. The Greeks were the first to write it down, and as with any of the peoples of the time embellished the stories they had heard. Now very few outside of select groupings know the legend, and very little if any of the true make up of them exist.
What is known was they lived in an island system of concentric land circles joined by bridges and boats. At the central island, was a temple to the gods and their royal palaces. Whether the gods themselves walked the Earth in those days, or whether the royal blood-line was seen as divine is unclear. What was clear was for the time they had a written language, the wheel and advanced mathematics. They studied a number of other languages according to legend, and the Old Ones & the Dragons knew of their existence, though very very few of the latter recalls where Atlantis was.
The dragons that do remember something tell that the Atlanteans were descendants of the First Men of Pangea, and closely related to the island state of Mu in the Pacific Ocean. It seemed neither side had any contact for centuries before both were sunk below the waves. According to some dragons, both had the knowledge & magic to pierce the veil between dimensions, and had started to explore more than just the Earth. They started to explore other dimensions and even other worlds of the Material Plane.
This had led them to come into contact with elements of the evil deities of Set & Apophis, with many of the nobility starting to worship them. Being highly magic, they summoned great destructive forces on their enemies and brought snake worship onto Earth. There they brought elements of the primordial chaos too, and placed knowledge in the minds of early modern man, especially the Phonecams & the Egyptians. Their own gods were horrified that they had done this, and demanded the Atlanteans to repair the damage they had wrought.
Unfortunately, their new found powers caused a great number to start changing into a new race, the Yuan-Ti. Their siblings in Mu had also started the same practises, along with elements who had changed in secret on the Old Kingdoms of Pangea. This started to affect the newer species too, and the gods lost all patience. Decrying this horrific corruption, the gods drowned both island nations in the one go below the seas to drown forever, and never looked back.
To this day,. both cities with Atlantis being the most well known, especially in Europe, lie below the sea and are regarded as mythical places with treasures before the New Race of Man made empires, and spread. Many an explorer and scholar search for these, to find these treasures, and to settle one other dispute. A few records of unknown origin, and reported to be written in a language called R'lyehian, report that it was the Old Ones or Ancient Ones the Atlanteans worshipped, at least initially. They are also depicted in these as trading with the Men or Ur for the secrets of dreams, and slaves of another type.

"I've heard the stories of lost Atlantis. I wouldn't put too much into them. Even if it's real, there's the fact that it would be at the bottom of the ocean. That, and the gods that cursed them to drown are the Old Ones, or the Ancient Ones, depending on the translation. Supposedly turned to worshipping other evil gods those ones."
Father Tossil speaking to other members of the Order of St Benedictus.