The Central 46 can sentence a Shinigami to be held in an underground prison for a set amount of time. The prison has eight levels which prisoners are confined in based on the severity of their crimes.
List of Prisons[]
Hikui (低い) is the highest level of the underground prison, and there is only one special feature than there is in other prisons: It's walls are made of Sekiseki. The sentences in Hikui are restricted from 1 month to 6 months, and are usually for those who have broken only a few rules that the Central 46 have issued, like killing a human.
Tsugi (次) is the next level down within the underground prison. With few recreational activities, it is the start of the prisons considered to drive some insane. It is much like a high security prison in the Human world, with guards all over, and little lighting. The sentence is between 6 to 12 months, for those who have done serious criminal offenses, like giving Shinigami powers to overs.
Shūgō (習合) is the 3rd level down within the prison. There is no lighting, and although the imprisoned aren't restrained in any way, they are considered restrained from humanity, most going insane through time, due to the minimum of a year's presence within, to a maximum of 10 years. This is usually given for those who have used forbidden Kido, and other such techniques which are illegal.
Kiken (危険) is the 4th level down in the prison. Here, even more restrictions are put onto the victim, as they are restricted from touch, and some low level Bakudo restrain them. This is usually the restriction for Human World imprisonments, being from 10 to 50 years of imprisonment, doing nothing but serving their sentence, for usually international crimes.
Seikatsu (生活) is the 5th level of the prison. Situated a mile underground, this is a prison where the most dangerous of criminals are situated, alongside dictators and others. An extra sense, sight, is removed and the target is bound under special tape around the eyes and mouth. This is the first prison where all power of the target is removed, and so the special tape covers the broken Soul Chain as well. Here, the sentence is from 50 until 100 years, making most go insane from lack of sight and touch.
Shi (死) is the 6th level of the prison. Situated 1 1/2 miles underground, it is where those that are convicted of treason are imprisoned, and yet another sense is taken from them, being hearing. Losing 3 of the 5 senses, it is almost guaranteed that those who are imprisoned go insane during just the minimum of 100 years, and 1,000 years being maximum. Here, Shi guarantees that the target is bound from the moment their sentence starts, to the end, bound by mid level Kido.
Yami (闇) is the 7th, and second lowest level of the prison. Situated 2 miles underground, Yami situates a single dark room, where only the highest Bakudo will restrain them, and will be restricted from all senses but smell. The sentence is from 1,000 to 10,000 years, for those who commit treason to Seireitei.
Muken is the 8th and lowest level of the prison, situated 3 miles underneath the compound. Here, it is just a single room, painted with a special black paint which shows as complete darkness. There is a single chair, where the target is imprisoned, where they are restrained using every Bakudo. All Barriers are around the walls and entrance, and only the Royal Guard can go in. Not even the Central 46 have even imagined such a place, which is only for those who have commited crimes against Soul Society). The miminum sentence is 10,000 years to infinity. It was made specifically for Azashiro Kenpachi and Kuruyashiki Kenpachi, after they were imprisoned for treason against the entirety of the Gotei 13. Sosuke Aizen was later imprisoned there.