No traveller to the Aether can fail to spot the Black City. It is one of the few constants of that ever-changing place. No matter where one might be, the city is visible. (Always far off, for it seems that the only rule of geography in the Aether is that all points are equidistant from the Black City.)
The Chant teaches that the Black City was once the seat of the Divine, from whence They ruled the Aether, left empty when mortals turned away from Them. Dreamers do not go there, nor do spirits. Even the most powerful demons and seraphs seem to avoid the place.
It was golden and beautiful once, so the story goes, until a group of powerful mage lords from the Zulkath Magocracy devised a means of breaking in. When they did so, their presence and sin defiled the city, turning it black. (Which was, perhaps, the least of their worries.) They became known as the maleficar, cast down and tainted with cursed blood: the first Archdevils. Their eternal ever present lust to reach the city damning their minds.
Bishop Midromel,
Imperial Cleric
Codex: The Black City