Hōgyoku

The (also referred as the Orb of Distortion) is a unique substance in the form of a small glowing blueish/purple 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 person's inner desires. It was invented roughly 101 years before the main events of the story, by ex-leader of the Shinigami Research and Development Institute, Kisuke Urahara.

Overview
Through his research, Kisuke Urahara sought a method by which a Shinigami’s soul could be strengthened. At some point, he came to the conclusion that this could be achieved if the soul were to undergo Hollowification. With this in mind, he went on to develop a material that would be capable of destroying the barrier between a Hollow and Shinigami, leading to the creation of the Hōgyoku. However, immediately after creating it, Urahara placed a seal upon it when he understood how dangerous it was.

History
Sometime before Urahara even conceived the concept of the Hōgyoku, Sōsuke Aizen had already come to the same conclusion, having created one of his own. Realizing that it was still incomplete, Aizen presented to it the souls of hundreds upon hundreds of Shinigami and Rukongai citizens, to no avail. In time, Aizen analyzed the research data concerning Urahara’s own Hōgyoku, learning that it too, was incomplete. With this in mind, Aizen sought to steal Urahara’s Hōgyoku, and to present it to his own. Roughly 101 years before the current time, Sōsuke Aizen put an experiment into motion that would put the true capabilities of the Hōgyoku to the test. With the help of his subordinates, he forcibly caused Shinji Hirako and a number of other Shinigami captains and lieutenants to undergo Hollowfication. In an attempt to reverse the damage Aizen had caused, Kisuke Urahara attempted to use the Hōgyoku on them. However, while it succeeded in stabilizing their souls, it failed to undo the Hollowfication. Framed for the incident by Aizen, Kisuke fled to the world of the living with the group of Shinigami, bringing his Hōgyoku along with him. Eventually, the group of Shinigami gained control over their newfound Hollow powers and became known as the Visored, rogues rumored to have gained Hollow powers through illegal means.

Sometime after this incident, Urahara sought a means to destroy the Hōgyoku, but to no avail. Out of desperation, he deactivated it and placed it deep within the soul of a Shinigami, Rukia Kuchiki, without her knowledge. By giving her a special Gigai, which would eventually turn her soul into that of an ordinary Human permanently, he hoped it would hide the Hōgyoku forever. . His plan failed however, when Sōsuke Aizen betrayed Soul Society to retrieve it – unbeknownst to Urahara, Aizen was the one who had deployed Rukia to the Human World in the first place. Aizen had delved further into Urahara's research, discovering two ways to extract an object from a soul: the first was to evaporate the soul with an immense amount of heat, with an instrument such as the Sōkyoku. Because the Sōkyoku was only used for the execution of Shinigami that broke the law severely, Aizen murdered the Central 46, and impersonated them with the special ability of his Zanpakutō, Kyōka Suigetsu. He was thus able to orchestrate the events leading to Rukia’s execution. However, this plan was foiled 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, where he used it to bolster the power of his Arrancar army.

At some point of time following the creation of Wonderweiss Margela, Aizen went on to embed the Hōgyoku into his own body. He reveals this to Ichigo Kurosaki during the Gotei 13’s battle against his Arrancar army at the fake Karakura Town. Not long after, Aizen engages Isshin Kurosaki in combat, who pushes him to the limit of his Shinigami self. Detecting this, the Hōgyoku fully awakens, causing his body to start to undergo a transformation.

Properties
The Hōgyoku is a unique substance in the form of a small, light blueish/purple spherical stone. It has the power to materialize a person's inner desires. Its power can turn a Shinigami into a Visored, 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
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 discovered a way to bypass this awakening period temporarily. The Hōgyoku can be fully awakened, if only for an instant, by allowing the Hōgyoku to fuse with one who has double the Reiatsu of a Captain-level Shinigami instantly. Aizen himself possesses such a level of Reiatsu, and therefore uses the Hōgyoku and begins to create his Arrancar army, consisting of Hollows granted Shinigami-like powers.

Powers
Desire Materialization: Though initially thought to solely possess the power to control the boundary of the two contravening existences, that of Shinigami and Hollows, this was proven incorrect. Rather, the Hōgyoku's true power is the ability to capture and materialize the hearts of everything that exists around it. However, this power is not without limitations. Not only must the target’s heart be in the vicinity of the Hōgyoku, but if the target does not essentially possess the power necessary to beget their desires, however, then it is impossible to manifest. Aizen claims that all the events that happened around Ichigo Kurosaki, Rukia Kuchiki and Kisuke Urahara were events materialized by the Hōgyoku itself, which possesses a will of its own.
 * Hollowfication/Arrancarification: While not truly an ability of the Hōgyoku itself, the orb took on this property because it is what Kisuke Urahara desired when he invented it. Through this power, the Hōgyoku could be used to bypass the barrier that separates Shinigami and Hollows. As such, it can cause Shinigami to undergo Hollowfication, and Hollows to undergo Arrancarification. These processes lead to the creation of a Hollow-Shinigami hybrid and an Arrancar, respectively.

Fusion: As demonstrated by Aizen, the Hōgyoku can be embedded into one’s body, almost fusing with it completely. In this state, the Hōgyoku instinctively protects its master, granting him immortality. It heals its master’s injuries almost instantly, in a manner similar to a Hollow’s high-speed regeneration. As the Hōgyoku’s will further understands its master’s heart, the fusion between them progresses, restructuring his soul and triggering a transformation.

Limitation

 * 1) 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.

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 Visoreds. 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 "Visored".
 * Although mostly not shown, Aizen uses the Hogyoku to allow potential Arrancar under his command to acheive far more of their potential power upon evolution to Arrancar at a higher rate of success than if done without the Hogyoku. Aizen is shown using the Hogyoku to conduct Wonderweiss' transformation into a full Arrancar.

Subjects Exposed To the Hōgyoku

 * Visored
 * Arrancar
 * Sōsuke Aizen
 * Kaname Tōsen
 * Orihime Inoue
 * Yasutora Sado