1. The inhabited caverns and chasms below the surface world.2. The main theme would be fighting for resources. The 'dungeons' of the Underdark would be the sacked and crumbling remnants of groups who had temporarily amassed enough resources to claw their way out of barbarism.3. A mega dungeon would represent a treasure trove of resources that the Underdwellers would invade in waves.
1. The Underdark is a mythical place, talked about, but never visited. It is so inhospitable and alien to visitors that the littlest thing could kill. It is a place even demons fear.2. I wouldn't bother with a map. I like Rod's idea of fighting for resources, I'm a fan of that even in the everlight worlds. I would establish 'civilized' or at least organized areas where creatures have some form of higher intelligence, but also have wilderness area where large predators battle for dominance. 3. The immensity of it and the gaming philosophy that it is not meant to be completely explored. And I am assuming that the ever dark is massive. That the creature of the everdark would have footholds within the dungeon, but still only know small areas.