Arkhosia
Arkhosia is a historical empire from the Nentir Vale setting of Dungeons & Dragons. It was the great empire of the dragonborn, ruled over by both Metallic Dragons and Chromatic Dragons in a rare and almost unthinkable alliance, but was ultimately brought low by a decades-long bloody war with the tiefling empire of Bael Turath.
Most of the details on Arkhosia come from the "Ecology of the Dragonborn" article in issue #365 of Dragon Magazine and from the "Player's Handbook Races: Dragonborn" mini-sourcebook.