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.
The Arkhosian dragonborn were master of the arts of blending magic with martial traditions, with extensive colleges training both swordmages and bladesingers for use on the battlefield. This would even pass on to some of their vassal states; Rethmil, a human nation-state, maintains a tradition of "knights" in the form of swordmages and bladesingers riding dragonnes, because of its time as a vassal to Arkhosia.
The primary Dragon Gods of Arkhosia were Io, Bahamut and Tiamat, with Bahamut being the empire's primary patron and Tiamat pushed from acceptable worship over the course of its life. It was also home to two unique faiths; the Arkhosian Imperial Church, which worshipped a triumvirate of Erathis, Ioun and Kord, and the Io's Children faith, which revered Bahamut and Tiamat simultaneously, but also downplayed their moral extremeness, presenting them as polar opposites that point towards a middle way that is neither good nor evil, but a draconic ideal.