Beast Realm (Seireitou)

The Beast Realm (畜生道, "Animal Realm") is a dimension separate from that is inhabited by various clans of animal-like  known as Beasts. It is depicted as a 'hell' where certain souls are sent based on the sins committed during their lifetime that are deserving of punishment, but were not sent to. However, some clans have managed to return to the Soul Society despite their banishment, such as the clan.

Overview
Similar to, the Beast Realm is not a world that was created by the , but rather it had always existed as a state of existence in the. When the world was split into its current form, much like Hell, the Beast Realm had been separated and became its own spatial dimension. What sets the Beast Realm apart from the other known worlds is that there is no gate that grants access to the dimension, such as the or. The only way to enter the Beast Realm is to be sent there upon death or to willingly bestow oneself with the "Chain of Sin" (罪咎の鎖, Tsumitoga no Kusari), which binds the soul to the Beast Realm; no different than how the are bound to Hell by the same chains.

Behind the Scenes
The concept behind the Beast Realm was briefly touched upon by in Chapter 556. It appears to be based upon the Animal Realm from Buddhist mythology.