Population: Capital city roughly 9 million
Ethnicity: racialy diverse, even including some more adventurous surface folks
Imagine an underdark city built into a large open cave ceiling, with lots of lights so it looks like a moon in a night sky. The city is supported by towers encircling it which are literally built in the cave ceiling. Exploding in all directions from the city are rope/suspension bridges all over like a spiders web, taking the underdark denizens to almost literally anywhere. Like Rome, all under dark roads lead to Chandelier. The area features its own beautiful waterfalls, and even freeflowing lava vents that keep it comfortably warm at all times of the year. They grow glowing mushroom farms along the ceiling and walls for much of their own food supply, but also raise under dark animals for meat.