Sōkyoku

The Sōkyoku (双) is a large halberd, which the Gotei 13 uses at Sōkyoku Hill to execute Shinigami that have severely broken the law. When released, it takes the form of a huge and magnificent Phoenix-like entity. As noted by Sōsuke Aizen, it is a punishment usually reserved for Captain-level Shinigami that have severely broken the law (thus, the punishment for Rukia Kuchiki was highly unusual, which Aizen also noted). The Sōkyoku is said to have power of 1 million Zanpakutōs and has the defensive ability to block Zanpakutō of equal power, and it can evaporate the soul of the one being executed by it. Sōsuke Aizen hoped to use this as a means to retrieving the Hōgyoku inside Rukia Kuchiki's body, but failed to do so. Before the Sōkyoku could kill Rukia, Ichigo Kurosaki appeared and blocked the strike with Zangetsu (something the observing Shinigami found astonishing, since Ichigo was blocking something as powerful as the Sōkyoku with only one Zanpakutō). It is then subsequently destroyed by Jūshirō Ukitake and Shunsui Kyōraku using the Shihōin Shield.

Appearances in other media
In Bleach: Shattered Blade, the central focus of the plot revolves around the shattered Sōkyoku shards. Many characters note that just holding one shard increases the spiritual power of the one holding it, as long as they are holding it. Arturo Plateado hopes to use the shards to help him increase his power.