Hōgyoku

The also referred as the Orb of Distortion, is a unique substance in the form of a small orb that was thought to be able to dissolve the boundaries between a Shinigami and a Hollow, granting either the powers of the other. However, Aizen explained that it actually materializes the hearts around it. In other words, it materializes a persons inner desires. It was invented roughly 100 years before the main events of the story, by ex-leader of the Shinigami Research and Development Institute, Kisuke Urahara.

Overview
During Urahara's time in the Research Institute, an answer was sought on how to strengthen an individual's soul, the answer being for a Shinigami to gain Hollow powers through Hollowfication. This led to the development of the Hōgyoku, though once he had created it, Urahara immediately placed a seal upon it when he understood how dangerous it was.

Urahara was forced out of necessity to first use the Hōgyoku roughly 101 years before the current time in order to save the lives of Shinji Hirako and a group of other Shinigami, who were undergoing forced Hollowfication caused by Sōsuke Aizen. Though the Hōgyoku failed to stop the Hollowfication, their souls were stabilized thanks to Urahara. They were outcasted from Soul Society and fled to the world of the living with Urahara, who brought the Hōgyoku with him and swore to find a way to reverse Hollowfication. Eventually, the group of Shinigami gained control over their new-found Hollow powers and became known as the Vizard, rogues rumored who "gained Hollow powers through illegal means".

Sometime after, Urahara sought a means to destroy the Hōgyoku, though all attempts proved to be ineffective. Out of desperation, he deactivated it and placed it within the depths of a person's soul, Rukia Kuchiki's soul. By giving her a defective Gigai, which would eventually turn her soul into a normal human soul permanently, he hoped it would hide the Hōgyoku forever. His plan failed, however, when Sōsuke Aizen betrayed Soul Society to retrieve it. Aizen delved into Urahara's research and discovered two ways to extract an object from a soul: the first is to destroy the soul, with such an instrument as the Sōkyoku, used for Shinigami executions. This failed when the Sōkyoku was destroyed by Jūshirō Ukitake and Shunsui Kyōraku, so Aizen resorted to the second method. Using a device invented by Urahara himself, Aizen extracted the Hōgyoku from Rukia's soul and retreated into Hueco Mundo.

Though he gained possession of it, the Hōgyoku was rendered dormant by Urahara, and needed time to 'fully awaken.' Aizen discovered a way to bypass this supposedly without the knowledge of Soul Society or Urahara himself. The Hōgyoku can be fully awakened, if only for an instant, by allowing the Hōgyoku to fuse with someone who possesses an amount of spiritual power surpassing a Shinigami Captain, that being Aizen himself. Aizen uses the Hōgyoku and begins to create his Arrancar army, consisting of Hollows granted Shinigami powers.

Later, Aizen has his Arrancar capture Orihime Inoue due to her special ability, her Shun Shun Rikka. He reveals to her that every time the Hōgyoku is used, its power deteriorates somewhat, and so he brought her to Hueco Mundo to repair it. As a sign of trust, he shows her the Hōgyoku itself, which inspires her to seek to negate its existence with her powers, rather than heal it. Aizen's reasoning for capturing her is later proved to be a ruse, as he brought Orihime to Hueco Mundo in the hopes of luring away some of the Gotei 13 captains so he may invade Karakura Town with less resistance. Aizen later reveals that he has embedded the Hōgyoku into his body. As the fight drags on, Aizen explains more about the Hogyoku, and his body, with the Hogyoku in it, undergoes a transformation.

Properties
The Hōgyoku is a unique substance in the form of a small, light purplish spherical stone. It has the power to materialize a person's inner desires. Its power can turn a Shinigami into a Vizard, or a Hollow into an Arrancar.

It is explained that though Hollows can naturally become Arrancar, the difference in power is small. Artificially creating Arrancar with the Hōgyoku is said to unlock a great deal more of said Arrancar's potential power.

According to Tessai Tsukabishi when he first witnessed it, the Hōgyoku has an overbearing power which is unlike anything else in Soul Society, and completely unrelated to Reiatsu. It has a presence which, according to him, feels as if it's trying to stifle his very existence with its power.

Activation
However, immediately after its creation, the Hōgyoku was placed within a strong seal and rendered dormant by its creator, Kisuke Urahara. Once Aizen gained possession of it, the Hōgyoku was released of its seal, but still needed time to awaken from dormancy. Aizen later discovered through releasing it from its 'prison' and touching it directly with his hand that this dormancy could be bypassed, something he imagined not even Urahara understood. By uniting the sleeping Hōgyoku with someone of higher than Captain class reiatsu, it can briefly be used as if in its awakened state.

Powers
Desire Materialization: According to Aizen, the Hōgyoku possesses a will of its own. Though initially thought to solely possess the power to control the boundary of the two contravening existences, that of Shinigami and Hollow, this was proven incorrect, as the Hōgyoku's "true power" is the ability to capture and materialize the hearts of everything that exists around it. Aizen claims that all the events that happened around Ichigo Kurosaki, Rukia Kuchiki and Kisuke Urahara were events materialized by the will of the Hōgyoku.


 * Healing: The Hōgyoku also has some healing properties, as it is able to heal the injury inflicted upon Aizen, who had embedded it into his body, by Ichigo Kurosaki. Aizen explains that the Hōgyoku is instinctively protecting its master.

Limitation
The ability of the Hōgyoku is to materialize the hearts within its vicinity. If the target does not essentially possess the power necessary to beget their desires, however, then it is impossible. In that sense, it can be called a "Power that guides in the direction of desire".

Aizen explains to Orihime that every time the Hōgyoku is used, it begins to deteriorate, which is why he needs her to heal it. However, this is later proven to not be his true reason for bringing her to Hueco Mundo. Whether the deterioration of the Hōgyoku is true or a fabrication remains to be seen.

Accounts of Use

 * Aizen claims that the reason Kisuke Urahara misunderstood that the Hōgyoku has the ability to manipulate the boundary between Hollow and Shinigami, is because that was his own desire, which the Hōgyoku generated.


 * Aizen claims the reason Ichigo Kurosaki inherited all the spiritual power of Rukia Kuchiki, who then lost her abilities as a Shinigami, is because she had not been rescued from the pain of killing Kaien Shiba.


 * Aizen claims the reason unique abilities were summoned within Yasutora Sado and Orihime Inoue was because they cursed their own powerlessness from the bottom of their hearts.


 * Aizen claims that he knew that its power was not the "Ability to manipulate the boundary between Shinigami and Hollow" as Kisuke Urahara said it was. If that was actually true, then Shinji Hirako and his allies should not have been able to become complete Vizards. The Hollowfication they underwent, while simultaneously being an experiment in pure hollowficiation, was also something done to confirm the power of the Hōgyoku. Through the hand of Kisuke Urahara, and consequently the invocation of the ability of the Hōgyoku, Shinji Hirako and his allies evolved to perfect "Vizards".

Subjects Exposed To the Hōgyoku

 * Vizard
 * Arrancar
 * Sōsuke Aizen