Hollowfication

Hollowfication describes what happens when the wall separating a Shinigami soul from a Hollow soul is torn down, pushing the original spirit past its natural limits. Whoever masters the change gains a dramatic surge in Reiryoku along with Hollow abilities, embodied by an inner Hollow that mirrors them in their inner world and by a unique mask that appears when the power is called up. A Hollowfied Shinigami is essentially the mirror image of an Arrancar: one is a Shinigami who gained Hollow power, the other a Hollow who took on Shinigami power.

The transformation can be triggered in more than one way. Aizen's engineered Hollow, White, spreads it through a bite; the Hogyoku can grant it outright, as it did to the Visored; and once transformed, a Hollowfied Shinigami can pass an infection through wounds, sickening the victim until they vomit up the beginnings of a mask. Because a plain soul cannot contain the process, only Shinigami of high spiritual power survive it. To wield the power deliberately, a Shinigami must defeat their inner Hollow within a duel that has to close inside roughly an hour; run past that mark and the Hollow wins control. Even mastered, the mask carries a time limit, commonly three minutes, and cannot be resummoned freely.

The payoff is a steep rise in strength, speed, and durability that Kaname Tosen ranked above Bankai, along with Hollow techniques like the Cero and, in some cases, outright Arrancar powers such as Sonido, high-speed regeneration, and even a Resurreccion. The gravest danger is Soul Suicide, the eventual self-destruction of a soul whose boundaries have been broken; Urahara's stabilizing vaccine spared the Visored from it, while Tosen, left untreated, violently exploded after his defeat.

The Visored are the definitive example, a band of Shinigami, Shinji Hirako, Hiyori Sarugaki, Kensei Muguruma, Mashiro Kuna, Love Aikawa, Rojuro Otoribashi, Lisa Yadomaru, and Hachigen Ushoda, forced into the state by Aizen and branded criminals for it. Ichigo Kurosaki carries the power as well, cycling through several escalating forms, and Kaname Tosen received it directly from Aizen. Urahara later weaponized the phenomenon through the Shin'eiyaku, briefly Hollowfying Shinigami so their stolen Bankai would poison the Quincy holding them.