Sōkyoku

The is a large naginata which the Gotei 13 uses at Sōkyoku Hill to execute Shinigami who have severely broken the law.

Overview
When not in use, the Sōkyoku is sealed by a number of thick ropes wrapped around its pole, which, piercing the ground, hold the halberd in place. During an execution, the Kidō Corps unseals it, causing these ropes to unwind and shoot off away from the hill.

As the Sōkyoku activates, emitting an immense amount of flames, it rises into the air, pointing towards the victim, who is held before it by support beams. The flames, enveloping the halberd, reveal its true form; a massive, phoenix-like entity called. In this state, the Sōkyoku’s offensive and defensive power is said to be equivalent to that of a million Zanpakutō. As it pierces the victim, this power increases by tenfold, and creates so much heat it vaporizes their soul.

Plot
In the letter he wrote to Momo Hinamori, Sōsuke Aizen claims Tōshirō Hitsugaya planned to use the Sōkyoku’s immense power to destroy Soul Society. However, Aizen later reveals he needed the heat it produced to vaporize Rukia's soul so he could retrieve the Hōgyoku within her.

When it is used for Rukia's execution, Ichigo Kurosaki, arriving, blocks it with his Zanpakutō, Zangetsu, an act which Suì-Fēng says should be absolutely impossible. The Sōkyoku, preparing to attack again, is destroyed by Jūshirō Ukitake and Shunsui Kyōraku using the Shihōin Shield.

Appearances in other media
In Bleach: Shattered Blade, it is revealed the destruction of the Sōkyoku broke the seal which was imprisoning a powerful Arrancar named Arturo Plateado. At the same time, the Sōkyoku shattered into several pieces, even one of which could augment the spiritual power of its holder. Arturo plots to collect all of these shards and use their power to decimate Soul Society.

Trivia

 * In the Anime, the Kidō Corps used hand signs to release the Sōkyoku.