The Hōgyoku (崩玉, Crumbling Orb; Viz: "Breakdown Sphere") 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.
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.