Yomi

Yomi, Naraka, Diyu, this place goes by many names, but regardless of what it is called by the mortals, all know it by what it is: the demon realm.
A place in which those souls judged to bear a karmic debt too great are sent for penance, to be cleansed and reincarnated through torment, Yomi and its denizens stand in a strange position with regards to the processing of souls. Though the demons bear a enough of a sense of law and honor to uphold their part of the cosmic duties, they constantly seek to subvert the ancient agreements where they can. It is the goal of many demonkind to gather souls of mortals, so imbued with magical power, and use them for their own ends. This, naturally, puts them at odds with Judges, but the two entities co-exist with a certain agreement that neither side will escalate things too far, leaving it at jockeying for individual souls for the time being.
Yomi itself, though, is a confusing and paradoxical land for those unfamiliar to it. Chaotic, yet orderly. Paradise, yet torment. The realm is a vast, seemingly endless existence divided into many domains held by the powerful. An eternal starless night sky hangs overhead with a shattered full moon, its myriad fragments and remaining body providing illumination all across the realm. Forests and deserts, mountains and tundras begin and end with seemingly no rhyme or reason, yet all seem co-exist.
Each domain is molded by its owner, imposing their own laws and wishes upon it. Some realms may appear as lush, beautiful gardens, while others may be realms of chaos in which time, gravity, and direction constantly warp and twist upon themselves. It is not uncommon for the different domains to war against one another, fighting for territory, pride, or some select prize that is sought after. However, one peculiar nature to these conflicts is that they are rarely viewed particularly seriously, and are often more treated as more of a friendly sport than a matter of life and death. Only when one's pride is on the line are these given some matter of grave import.
In the alleged center of Yomi is one such domain: Izani. Maintained as a neutral territory where the only violent conflict permitted are officialized duels, it is both where mortals first arrive and where a majority of demons not sworn into service of another gather, and serves at the largest domain in Yomi, functioning as a colossal city well over the size of many countries. Here, souls of mortals that have been judged are processed into their respective torments, while those claimed through demonic dealings are exchanged for a myriad of dark purposes. Two particular features of note are a massive hearth, where mortal souls that arrive are treated to a "last meal" before their penance begins, and a hot spring of striking purity, where it is alleged that one could wash away all forms of corruption in its waters.
Beyond the territory of Izani, the rules and laws in place are almost nonexistence, only that which is imposed by powerful demons that have claimed regions for themselves. However, there is one thing that is agreed between all of demonkind: You may fight one another, take territory, and more, but one should never induce Mashi, True Death. Being immortal creatures that simply regenerate upon being struck down, Mashi is the only form of real, lasting consequence that a demon could truly face. To date, there have only been 9 instances of it being performed.