Hōgyoku

The also referred as The Orb of Distortion, is a unique substance in the form of a small orb that can dissolve the boundaries between a Shinigami and a Hollow, granting either the powers of the other. It was invented roughly 100 years before the main events of the story, by ex-leader of Shinigami Research and Development Institute, Kisuke Urahara.

History
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, a substance which can shatter the boundaries between Shinigami and Hollow in an individual instantly, creating a new existence.

Urahara was forced out of necessity to first use the Hōgyoku roughly 100 years before the storyline 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 Kisuke, 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, Kisuke realized how dangerous the Hōgyoku truly was, and sought a means to destroy it, 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 Ichigo Kurosaki, so Aizen resorted to the second method. Using a device invented by Kisuke 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 Kisuke, and needed time to 'fully awaken.' Aizen discovered a way to bypass this, fully awakening it for an instant by allowing the Hōgyoku to fuse with someone who possesses at least twice the spiritual power of 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 Inoue Orihime due to her special ability, her Shun Shun Rikka. He reveals to her 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 in actuality 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 Inoue to Hueco Mundo in the hopes of luring away some of the Gotei 13 Captains so he may invade Karakura Town with less resistance. Whether the Hōgyoku truly deteriorates after each use is not ever really confirmed.

Trivia
When in use, the Hōgyoku sprouts small appendages that enter the user's skin. All of this bears a striking similarity to the action taken by a Ring of the Dead when it comes in contact with a person's skin in Tite Kubo's manga, Zombie Powder.