8th Level: Muken

Muken (無間, Avici) is the eighth and final level of the underground prison. Sōsuke Aizen was sentenced to 18,800 years in this level for high acts of treason and a long list of other crimes. His condescending remarks during his trial raised the sentencing to 20,000 years.[2] Muken is a voided space completely sealed off from the outside world, aside from the main tower that serves as the entrance and exit. The realm stretches on infinitely. Under normal circumstances, non-criminals are not permitted entrance, but Retsu Unohana managed to gain permission from Captain-Commander Shunsui Kyōraku to use the location as a battlefield to train Kenpachi Zaraki in Zanjutsu.[4] Muken is reserved for criminals whose heart cannot be stopped by any means by executioners (i.e. those who are immortal).[5]