The Kingdom of Skyy
Heirs to an ancient, infernal bloodline, tieflings have no realms of heir own but instead live within human kingdoms and cities. They are descended from human nobles who bargained with dark powers, and long ago their empire subjugated half the world. But the empire was cast down into ruin, and tieflings were left to make their own way in a world that often fears and resents them.
Source: Player’s Handbook