Sōkyoku

The Sōkyoku is a large polearm, 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).