Makai

Makai(AKA the Demon Realm) is a plain of existence located outside of our own and is the home of all demons. The layout of Makai consists of 9 cirlces with each having their own hierarchy & their own leader. The 9th circle is composed of small clique of S-Rank demons with incredible power with only one of them being able to ravage the entire human realm with little difficulty. The 1st circle is filled with the most insignificant demons who are not even considered strong enough to be considered cannon fodder.

When a mortal dies, their soul is judged by the Shinigami and they will either be damned or given permission to enter the next stage of judgement. If they're damned, they will become a demon with their level of sentience, species, and strength depending on on their intelligence in life, personality in life, purity in life, beliefs in life, actions in life, and countless other aspects coinciding with the three traits of existence(Mind, Body, and Soul).

In the very beginning, the 9 circles were not fully independent as they were united under a single banner by 3 S-Rank demons: Yomi, Mukuro, and Raizen. This changed when the 3 kings died and the great collapse occurred.