The Obsidian City is a dark negative version of the Emerald City, in an alternate reality.
The City is built out of shining black volcanic glass. Its massive architecture is impressive, but gloomy and ominous. The glass in the windowpanes does not glitter.
The people dress in black rags, and sport dirty faces; their manner is furtive and suspicious of others. They move sluggishly, and seem universally poor, discontented, and miserable. The sounds of weeping and moaning can be heard in the streets.
When Ozma visits the City, she meets alternate versions of long-familiar people. Omby Amby, the Soldier with the Green Whiskers, here calls himself Wantowin Battles; his long beard is, of course, black. Wantowin arrests Ozma and forces her to wear black glasses as she walks through the city streets (just as Dorothy Gale had to wear green-tinted glasses when she first visited the Emerald City).
The City is ruled by a tyrant, the Great and Terrible Wizard of Oz, from his giant black throne. He is assisted by his headsman, Nick Chopper. (Paradox in Oz)