Labyrinth

The Labyrinth is an underground city of monsters and also the home of the Black Airs. Humans generally are not allowed in Labyrinth, However they can easily sneek in as shown by the Vargan Brothers, thus revealing that it is a loosely guarded city. Humans tend to stay away for fear monsters will eat them and monsters don't really venture out for fear human will kill them, in reality it's a big circle of unnecessary fear of each other.