Hollowfication

is the term used to describe the process by which the strength limit of a soul is increased by removing the boundary between Shinigami and Hollow. This allows one to gain tremendous power if their Hollowfication can be controlled, which is personified by an "inner Hollow" reflection of the person in their inner world. A Hollow-like mask appears when entering this state. The term is synonymous with the act of donning a Hollow mask.

Overview
Hollowfication is the act of inserting a Hollow's soul into a normal soul, then breaking the boundary between them to try and turn the original soul into something more powerful. Originally this was meant to be used to strengthen a Shinigami's soul, but the technology that Soul Society possessed was not advanced enough to control it.

When a soul is "Hollowfied", as the condition progresses the original soul gets mixed up with the Hollow soul, becoming a monster unable to reason. In the end not only the boundary between souls, but also the soul itself and the external world is disrupted and the soul, regardless of its own will, self-destructs. This is called.

According to Kisuke Urahara and Mayuri Kurotsuchi's hypothesis, a Quincy is incapable of Hollowfication, as Hollow powers are essentially "poison" to them, and should a Quincy be infected, it would weaken their spiritual powers and lead to a Soul Suicide, with no chance of Hollowfication to occur.

Hollow-Shinigami Hybrid
By gaining the strength of a Hollow, one's Reiryoku increases dramatically. Only those who have control over their inner Hollow can Hollowfy. It manifests itself as a unique mask of different shapes and designs.

When Ichigo's Hollow powers started to increasingly manifest while not under his control, Shinji explains Ichigo's conflict with his inner Hollow is a common symptom other Visored had to deal with at one point or another in order to master their abilities.

Creation Process
The process by which to activate such a transformation can be achieved by three possible methods, though only two are known in any real detail. In any process, the main determination of having achieved Hollowfication is the manifestation of a Hollow mask.

White
A creation of Sōsuke Aizen formed from the souls of many Shinigami. White is capable of infecting whomever it bites with a poison that induces Hollowfication. His initial creation was for the purpose of infecting a Shinigami and turning him/her into a Hollow. By entering a final "transfer" stage, it can also enter an individual's soul and fuse with their latent Shinigami powers. Despite its creator's intentions, White chose a Quincy as its target, activating its self-destruct mechanism.

Unknown Infection
It is unclear how the other Visored gained their current status. What is known is Sōsuke Aizen was instrumental in the Hollowfication process, though whether he knew of the full potential of the results is left to be questioned. The process used seems to spread as some form of possible infection through wounds which has the desired effects on souls with high-level Reiryoku, as compared to souls with low-level Reiryoku. The infected experience great sickness and fatigue, eventually vomiting up Reishi in the form of a white liquid, which forms the beginnings of their Hollow mask before proceeding to transform their bodies and warp their minds. Other than that, all that is known is the process is accelerated by increased stress. The conclusive result makes a Shinigami into a primarily Hollow-like humanoid, which is a highly unstable transformation causing mindlessness and aggression.

Hōgyoku
The Hōgyoku possesses the ability to transform a Shinigami into a perfect Hollow-Shinigami Hybrid.

Kisuke Urahara's Hōgyoku played a vital part in the process of making the Visored currently what they are, as he tried to use it to return them to their previous Shinigami state. According to Urahara, what his device did was not what he had intended; instead, the Hōgyoku stabilized the transformation into the hybrids and he considers the solution a failure on his part. They did not become complete Hollows, due to his intervention, and did eventually gain control over their new power.

The Hōgyoku is implied to have had a hand in turning Kaname Tōsen into a perfect Hollow-Shinigami Hybrid. While maintaining all the added benefits of a Visored transformation, Tōsen is notably different in the regard that he not only possessed a Hollow mask, but a Resurrección, allowing him full access to a true sealed away complete Hollow form. The circumstances behind this are yet to be explored in detail to determine if this is a unique situation, or just something not yet seen among the Visored.

Solution
The only way to stop the "Soul Suicide" is to insert into the soul something opposite to Hollowfication. The destruction of the boundary between souls, which is the key to Hollowfication, happens by disrupting the balance of the souls. In other words a conflicting object would restore the balance on the other side and stop the destruction. There are only two known solutions to Hollowfication both developed by Kisuke Urahara.


 * Hollowfied Shinigami: Developed to stabilize the Visored. By creating a vaccine using the Quincy Heilig Pfeil and Human souls and injecting them into the souls of the Shinigami that had been Hollowfied, he was able to stop "soul suicide" in all of them.


 * Hollowfied Quincy: Developed to stabilize Masaki Kurosaki. By using a specially created Gigai made from a human soul that envelops the Shinigami who enters it, becoming like a human body. If a Shinigami enters it then they will become a being midway between Shinigami and Human. They will become the opposite to that which they are attempting to stabilize (In this instance a Human as they are attempting to stabilize a Hollow). The drawbacks of this is that while inside the shell the use of Shinigami powers would be impossible nor will they be spiritually aware. Upon entering there is also no guarantee that the Shinigami will be able to go back to being a Shinigami again. Finally to make the shell work as a vaccine to block Hollowfication both the soul and the Hollow inside must be bound with a Reishi string. The string is extremely strong and its purpose is to bind souls, as long as its connected the Shinigami won't be able to leave as long as the subject lives.

Those who have undergone Hollowfication

 * Visored: The most well-known group of Shinigami who have undergone Hollowfication are the Visored. There are eight members of the Visored, whom were all either former Gotei 13 captains or lieutenants (including Hachigen Ushōda, who was lieutenant of the Kidō Corps, and Substitute Shinigami Ichigo Kurosaki). As Aizen stated, "a normal soul can't contain the prototype" when undergoing Hollowfication, therefore Shinigami of a high spiritual level have to be the ones who undergo this process.


 * Kaname Tōsen: Former captain of the 9th Division. Turned traitor with Sōsuke Aizen and given Hollowfication for his loyalty. He is noteworthy for being the only known Hollow-Shinigami hybrid with a Resurrección.

Other Situations
When the Wandenreich used their Medallions to steal the Bankai of the Gotei 13, Kisuke Urahara hypothesizes that a Quincy cannot intake Hollow Reiatsu without suffering injuries. To counter the Bankai-stealing, Urahara invented the Shin'eiyaku, which temporary Hollowfy the Shinigami taker, thus poisoning the Quincy who are in possession of the respective Shinigami's Bankai.

Appearances in Other Media
In Bleach: Fade to Black, the beings 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 forced into submission rather than a completely new Hollow being spawned from their consciousness.

In Bleach: The Hell Verse, Hollowfication is noted by several of the Togabito as being capable of liberating them from Hell. The Togabito Kokutō notes the powers of Hollowfication are influenced directly by the atmosphere of Hell, in which, if left uncontrolled, their inner Hollow could proceed to consume them entirely. This is most notable when Ichigo Kurosaki's Hollow mask manifests itself when he attempts to release a Getsuga Tenshō attack.