Legend has it that thousands of years ago, before any of the known races attained anything that could remotely called civilization, the climate was different and the seas levels much lower. In those days the entire archipelago of The Thousand Isles was actually one large continent, and one could walk from the South Reach to the North.
This continent was ruled by an ancient empire of snake like creatures, the Yuan-Ti. Before they supposedly perished in a Great War with the dragons, they were said to have been early teachers of mankind. Teachers, rulers, and owners of mankind.
No one knows much about this lost civilization, or what caused the Deluge that destroyed it and sunk the lands below the waves They are mostly remembered through their ruined pyramids and stone work can still be discovered in isolated locals throughout the isles. Outside of The Thousand Isles, their influence is much more apparent in Great Mbo.