Days of Thunder

Tens of thousands of years ago, empires of reptilian, amphibian, and avian peoples — known in Elvish as Iqua’Tel’Quessir, the creator races — dominated the world. They built great cities of stone and glass, carved paths through the wilderness, tamed the great lizards, worked mighty magics, shaped the world around them, and warred upon each other. Those were the Days of Thunder.
The age of the creator races came to a sudden end some thirty thousand years ago. Perhaps their wars reached a terrible and inevitable crescendo, or they tampered with forbidden forces. For whatever reason, the world changed, and their vast empires vanished. All that remains of them are ruins and the scattered lizardfolk, bullywug, and aarakocra tribes, barbaric descendants of those who once ruled the world.
History of Faerun