Hollowfication

Hollowfication is the term used to describe when a Shinigami gains Hollow powers, which creates a Vizard. In doing so, the Shinigami's strength greatly increases as well as their agility, speed, stamina and pain tolerance. In addition, the Shinigami may also obtain access to Hollow powers such as Cero and Sonído. The most well-known group of Shinigami who have undergone Hollowfication are the Vizard. There are eight members of the Vizard, whom were all either captain-class or lieutenant-class (excluding Ichigo Kurosaki, who they count as one of their number). As Aizen stated, "a normal soul can't contain the prototype" when undergoing Hollowfication, therefore Shinigami have to be the ones who undergo this process.

After a Shinigami obtains Hollow powers, they must don their Hollow mask in order to access them. The mask eventually crumbles and disappears after a certain period of time, which varies for each Vizard. For example, on her first try, Mashiro Kuna could maintain her mask for at least 15 hours without tiring. On the other hand, Ichigo could only maintain his mask for four seconds at first, though he managed to train enough with the Vizard to maintain it for a confirmed 11 seconds prior to his second battle with Grimmjow Jaegerjaquez. This is the reason they go under special training to build up their stamina so they could hold their masks for a longer period of time. The appearance of each Hollow mask is different to the individual.

The Vizard, with the exception of Ichigo, fell victim to Hollowfication as a result of Sōsuke Aizen's experiments about 100 years before the current storyline to create a Shinigami/Hollow hybrid, and were stabilized in their hybrid forms by exposure to the Hōgyoku. It is unknown how Aizen achieved this process but it was considered a failure because they weren't perfect hybrids. Ichigo, on the other hand, obtained his Vizard powers differently. As a result of regaining his Shinigami powers while nearly becoming a Hollow, the Hollow in the process of being created had spawned within his conscience. After eventually subjugating his inner Hollow, Ichigo is finally able to don his Hollow mask like the other Vizard can.

Appearance in Other Media
In Bleach: Fade to Black, the characters of Homura and Shizuku underwent a different form of Hollowfication, absorbing a Hollow into themselves, and later merged with Rukia Kuchiki to create the hybrid being known as Dark Rukia. A similar form of Hollowfication occurs in Muramasa during the Zanpakutō Unknown Tales arc, as a result of absorbing Hollows to sustain his existence. Both of these forms involved pre-existing Hollows being merged with the individual in question and then forced into submission, rather than a completely new Hollow being spawned from their consciousness.