The City of Onyx is the capital of the Khayal genies. A city of twisted spires and tall minarets growing in all directions in defiance of gravity and the laws of physics, it is located in the Plane+of+Shadow.
Members of thousands of races throng in the City of Onyx's bustling marketplaces. The city is a popular stop among travelers using the Plane of Shadow to cross worlds. Foreigners are not permitted in the city's core, however, which is off-limits to all non-khayal.
The ruler of the City of Onyx, and the khayal race, is the Malik al-Khayal, the so-called King of Shadows. Lesser khayal nobles generally claim the rank of emir.
Source: Tome of Magic (D&D 3.5)