Jōkaishō

A Jōkaishō is a massive, powerful device that was once used by scientists in the Seireitei to power their experiments and allow them to control the generation of Reishi.

Overview
A Jōkaishō is a massively powerful device that can be used for various purposes but was most commonly used to help scientists, like Ran'Tao, to have a degree of control over their experiments. The common use of the Jōkaishō was to allow the scientists to control the generation of Reishi so they could implement this into their experiments to give them a higher degree of success otherwise. Due to this, dozens were scattered all across the Seireitei to further their goals.

History
Many centuries before the birth of Ichigo Kurosaki, within the Seireitei there were a group of scientists that long predated the existence of the Shinigami Research and Development Institute. These scientists, led by Ran'Tao, were exploring various ways to achieve eternal life and had numerous successes in advancing towards this goal. During these investigations, they used samples of their own souls to experiment on and managed to create their own artificial souls by encompassing these samples with aspects of Human souls. These exploratory and advanced experiments were considered to have an overall high success rate at the time so several studies were taking place at once, all over the Seireitei.

However, at some point, an accident happened that caused the accidental creation of the Bount in the World of the Living. These trials and experiments were being powered by many Jōkaishō which also allowed the scientists to have massive control over the results of the tests. Despite the predictions of high success, one of the Jōkaishō became unstable and exploded, causing the accident that resulted in the birth of the Bounts. Not only were the artificial souls created by the experiments dispersed into the stream of souls being reborn but the explosion also destroyed a tenth of the Seireitei, causing widespread damage and loss of life. As a result, the experiments were immediately halted and efforts focused on rebuilding from all the destruction.

Following this massive destruction, Central 46 decided that the Jōkaishō, while useful in experiments, were too unstable and destructive to be left unchecked. However, it was too dangerous to try and destroy them and this could cause the cataclysmic destruction of the Soul Society. Thus it was decided that the Jōkaishō were to to be safely sealed away and this was done by the Kidō Corps a long time ago. When it became clear that the accident had caused the creation of a new species of Human, Central 46 decided to bury and destroy all information and evidence related to the experiments and this included knowledge of the Jōkaishō. However, sparse data on the survived in the libraries but it was overlooked for centuries as nobody ever looked for it.

In the present day, Jin Kariya has managed to learn of the history of his race. He uses the unique talents of Maki Ichinose, who can use his Zanpakutō to render himself invisible, to frequently infiltrate the Seireitei to gather and destroy information as he sees fit. Having learnt of the existence of the Jōkaishō at some point, Kariya decides that they are central to his plan to avenge the Bount and their suffering by using the Jōkaishō to destroy the Soul Society. After invading the Seireitei, he attacks a compound of the Shinigami Research and Development Institute and destroys much of it to access a Jōkaishō that is sealed beneath it. Once he finds it, he absorbs its power into himself so that he can utilise it in his final plan.

The Gotei 13 manages to learn that it takes a full day to fully charge the powers inside the Jōkaishō so they decide to put this time to good use and find out as much information as they can from their archives. Jūshirō Ukitake is put to work in the libraries and by sheer luck manages to find one of the books written about the Jōkaishō that was authored by Ran'Tao. Realising that many more books must be stored but scattered around the library, he enlists the aid of Shunsui Kyōraku to find more before time runs out. Gō Koga stands up to Kariya as he doesn't believe the Jōkaishō should be used to destroy the Soul Society but to force the Seireitei to help create a better paradise for all but Kariya disagrees and abandons him.

Having realised Kariya's plans after confronting him, Ran'Tao manages to impart her knowledge of the Jōkaishō and where they are to Tōshirō Hitsugaya once she confirms that he is a captain within the Gotei 13. Kariya plans to use his Jōkaishō to destabilise the dozens of Jōkaishō scattered across the Seireitei to cause a cataclysmic event in the Soul Society. Ran'Tao informs Hitsugaya that while the Jōkaishō cannot be destroyed, they can be re-sealed again to protect them; their original seals have become weakened over time and this has made them vulnerable to Kariyas's plan. During his battle with Ichigo, Kariya tried to blow up one of the Jōkaishō to demonstrate its power but his attack is intercepted by Byakuya Kuchiki who arrives to bide time for the rest of the Gotei 13 who are currently resealing the Jōkaishō. Kariya's plan ultimately fails as he is defeated by Ichigo, which ends the threat posed to the Jōkaishō and they are resealed by the Gotei 13 as well.