The Onyx Empire once occupied almost all of the MidLands except Shael i'Vor as well as a good part of the NorthLands. It is so named because onyx was the favored stone of the wizard Keraptis, who founded the empire some five hundred years ago after his betrayal of the Purple Castle. The name was also used to describe its mood -- black, like the stone of its name.
The empire was conceived in the city of Arek, capital and largest city of the then-pastoral country of Arin in the geographical center of Lanad i'Sarois. From there, it grew outward to encompass a third of the continent, while its allies inhabited a good deal of the rest.
Eventually, Keraptis' forces were defeated by the White Alliance and other armies in the Wars of Arin, and Arek itself was nearly wiped from the map. Although one of Keraptis' minions, the wizard Morjec, is still the power behind the throne in what remains the country of Arin, the empire is no more.