Both the city of darkness and the dwelling place for demons is what the Alliance is. For the Alliance itself is where an elite race of demons live. However, the creatures there are not the demons that most people encounter and that angels can easily destroy, because the darkness that these demons consist of is entirely different. The Alliance itself is located in a world created by it's leader. In a world found on the other side of the mirrors. In this world the leader, Kyoko, is able to control everything and is able to manipulate the darkness there that is unknown to the world on the other side. Doing so creates a species that no one can conquer. Not only that but she also ensures that none of the members of the Alliance can be killed, and in order to do that she kills them herself.
Each member of the Alliance is a person who once existed but was killed. Once killed Kyoko would possess them with a demon of sorts that would bring them back to life and would be inevitably invincible in a sense. Of course these demons can still feel pain, and their weakness is usually emotion. However, light doesn't hurt them severely considering that their darkness usually engulfs the light before it can touch them.
The Alliance itself is not just a mere building but an actual city. In this city lies places where people are tortured and trained to become an elite demon. Others are where the dwelling places of these demons lie. Only those who are apart of the Alliance can enter this eerie place, and only those who are apart of the Alliance would want to.
[I'll add more later. I'm just lazy.]
