Purgatory

Purgatory was specifically created by the Mei-flower to restrain and hold the criminals and other invaders to Niflheim, however it's a realm where the souls of monsters go after they die. Here, the monster souls are fated to hunt and prey upon each other for eternity. Since then, it has become home to various other creatures.

Here is some example of crimes that can get you trapped in the realm of Purgatory: Treason and Solus.

Jennifer and Tobias are examples of Ultimatum, the judgement of one who is accused of breaking all laws in Niflheim.

Purgatory is a place to be feared among those in Niflheim. There is no power here, to gain power you must devour another to the point where you get stronger. Since it's creation there has been an influx of creatures added to the "Prison." Each creature added thought they were the best, they could do whatever they want. They were cast into purgatory for their insolence and since then thousands of different species have been added. Humans, Demons, Angels, gods, Aliens, Vampires, Werewolves, shapeshifters, ghost, etc. Rose has stated that there is one person you do not want to be trapped in purgatory with but didn't state who it was. Jenova pointed out that she had stared at the Crystalized Iris.