Sōkyoku

The is a large halberd, which the Gotei 13 uses at Sōkyoku Hill to execute other Shinigami that have broken the law severely. When released, it takes the form of a 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 be as strong as 1 million zanpakutōs, and it evaporates 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. 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.

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.

Soul Society Arc
The Sōkyoku is probably the most powerful Zanpakutō in Soul Society. So much so that it completely dissipates a soul's spiritual body, and was part of Aizen's plan to obtain the Hōgyoku. This plan failed however, when Shunsui Kyōraku with Jūshirō Ukitake destroyed it, while Renji escaped with Rukia. It has yet to be seen or spoken of since.