Hōgyoku

The is a small, bluish-purple orb composed of a unique substance thought to have been capable of dissolving the boundaries between Shinigami and Hollow, allowing one race to attain the powers of the other. However, its true power lies in its ability to sense the hearts of those around it and materialize their deepest desire.

Two Hōgyoku are known to exist, the first invented by Sōsuke Aizen and the other by Kisuke Urahara. Ultimately, Aizen merged his orb to the latter's, forming a single Hōgyoku.

History
Some time before Urahara even conceived the concept of the Hōgyoku, Sōsuke Aizen, having already come to the same conclusion, had 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, by analyzing the research data concerning Urahara’s own Hōgyoku, Aizen learned that it was also incomplete. With this in mind, Aizen sought to steal Urahara’s Hōgyoku and present it to his own.

Roughly 101 years ago, Sōsuke Aizen put an experiment into motion which 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 used 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, gaining control over their new-found Hollow powers, became known as the Visored, rogues rumored to have gained Hollow powers through illegal means.

Plot
Sometime after his exile, 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 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, which could be accomplished with an instrument such as the Sōkyoku. Because the Sōkyoku was only used for the execution of Shinigami who broke the law severely, Aizen, murdering the Central 46, impersonated them with the special ability of his Zanpakutō, Kyōka Suigetsu, thus orchestrating 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.

Later on, Aizen has his Arrancar capture Orihime Inoue because of her special ability, the Shun Shun Rikka. He reveals to her that every time the Hōgyoku is used, its power deteriorates somewhat, and that 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 proven 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.

At some point after showing the orb to Orihime, Aizen combines it with Urahara's Hōgyoku and embeds the product into his own body. During the Gotei 13’s battle against his Arrancar army at the fake Karakura Town, he reveals this to Ichigo Kurosaki. 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 begin undergoing a transformation. Aizen is soon betrayed by Gin Ichimaru, who after dealing a crippling blow to him snatches the Hōgyoku and attempts to escape. However, Aizen transforms yet again, declaring that whether it is with him or not, the Hōgyoku is already his. As he says this, the Hōgyoku proceeds to melt away, reforming in Aizen's chest.

Moments later, a newly empowered Ichigo arrives and begins to fight the transformed Aizen. After being dealt a powerful blow by Ichigo, Aizen yells out in frustration, prompting the Hōgyoku to force yet another transformation upon him. Aizen, in a monstrous form, states that the Hōgyoku has no intention of allowing him to lose to a "mere Human." Regardless, Ichigo uses the Final Getsuga Tenshō and attacks Aizen with Mugetsu. Aizen survives the blast, regenerating from a large wound dealt to him, but remains heavily damaged. No longer recognizing the weakened Aizen as its master, the Hōgyoku causes his Zanpakutō to disintegrate, though he mistakes it for thinking he no longer needs one. Following Kisuke Urahara's arrival moments later, the Hōgyoku proceeds to reverse Aizen's transformations, returning him to his original state. Ichigo later suggests that Aizen reverted to his original form not because the Hōgyoku rejected him, but because Aizen secretly wished to lose his power and become an ordinary Shinigami.

Activation
When Aizen gained possession of Urahara's Hōgyoku, the seal the latter had placed on it was broken, but the orb still needed time to awaken from its dormancy. However, Aizen discovered a way to temporarily bypass this awakening period. He found the Hōgyoku could be fully awakened, if only for an instant, by allowing it to momentarily fuse with an individual who possesses at least twice the Reiryoku of a captain-level Shinigami. Aizen himself possesses such a level of reiryoku, and therefore uses the Hōgyoku to enhance his Arrancar army.

Powers
Overbearing Power: When Tessai Tsukabishi first witnessed the Hōgyoku, he stated it possessed an overbearing power unlike anything else in Soul Society, a power completely unrelated to Reiatsu. He goes on to say its very presence felt as though it were trying to stifle his very existence.

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 absorb the desires of those around it and manifest them into reality. However, this power is not without limitations. The Hōgyoku itself is merely a guiding force - it can only manifest the desires of those with the strength to carry them out. The events which 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 which 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, fusing with it almost completely. In this state, the Hōgyoku instinctively protects its master by healing 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.

Subjects Exposed to the Hōgyoku
Because of its ability to manifest the hearts of those around it, even if they are not aware of it, the Hōgyoku has affected several different individuals. In some cases, its powers were used intentionally.


 * Visored: When Shinji Hirako and seven other captains and lieutenants began to undergo Hollowfication because of Aizen's hollowfication experiments, Urahara sought to reverse the process by using the Hōgyoku. However, instead of undoing the process, the orb stabilized their souls, allowing them to become the Visored.


 * Arrancar: Though Hollows can become Arrancar through natural means, the difference in power is usually small. Artificially creating Arrancar with the Hōgyoku, however, is said to unlock a great deal more of said Arrancar's potential power. Wonderweiss Margela was among those empowered in this way.


 * Sōsuke Aizen: After using the Hōgyoku to bolster his army's power, Aizen implanted it in himself, attaining regeneration as a result. When his soul later reached its limit as a Shinigami, the orb caused Aizen to undergo a series of transformations, causing himself to transcend the boundaries between Shinigami and Hollows.


 * Rukia Kuchiki (Alleged): According to Aizen, she still held the guilt of killing Kaien Shiba. The Hōgyoku caused her to transfer all of her powers into Ichigo Kurosaki instead of half as she had intended.


 * Kaname Tōsen (Alleged): As a reward for faithfully serving Aizen, Tōsen was granted Hollow powers. As a result, he could don a Hollow mask and use a Resurrección, Grillar Grillo.


 * Orihime Inoue and Yasutora Sado (Alleged): According to Aizen (because Orihime and Sado cursed their powerlessness from the bottom of their hearts), the Hōgyoku caused them to develop unique abilities, the Shun Shun Rikka and the Brazo Derecha de Gigante respectively.