Location from Bleach by Tite Kubo
The deepest underground prison beneath Seireitei, reserved for the most dangerous criminals in Soul Society's history — currently holding Sōsuke Aizen bound to a chair for 20,000 years.
Muken is less a prison and more a concept — an endless void beneath Seireitei designed to contain beings too powerful to kill and too dangerous to leave accessible. Aizen was sentenced to 20,000 years here after his defeat, which Central 46 actually reduced from the original sentence because Aizen mocked them during the hearing. The irony is that Aizen — immortal thanks to the Hōgyoku — will outlive his sentence easily. During the Quincy war, Shunsui Kyōraku descended to Muken to negotiate with Aizen, temporarily releasing him as an ally of convenience against Yhwach. Muken is the Gotei 13's admission that some problems can only be delayed, never solved.
An infinite black void with no visible floor, walls, or ceiling. A single chair sits in the center of nothingness, bound in sealing straps. Aizen sits in it, partially restrained, his reiatsu suppressed by layers of kidō bindings. The silence is absolute — no sound reaches this depth.
Also known as: Muken, The Infinite Void, Central Underground Prison