Fiction:The Firmament/Black Metropolis

The Black Metropolis is the largest realm of the plane of Irkalla and the domain of the demon lord Xel'manikh.

Description
Believed to be as large as an entire mortal world, the Black Metropolis is an immense city of black, obsidian and adamantine towers where demons congregate and live sinful lives. At its center lies the Sorcerous Tower of Black, the personal abode of Xel'manikh from which he oversees the landscape. At the sewers of the realm lies Festerblight, the dominion of the demon lord Skanqrak. Looked from above, the Black Metropolis has an eerie resemblance to Telamon's continents put together.

Inhabitants
Beyond the nigh infinite amount of demons and evil souls who inhabit the landscape, the Black Metropolis is also the most common place for planar travelers to arrive to when they reach Irkalla. All manners of dark outsiders do business in the city, using souls as currency when bartering with demons.