Metastacia

is an experimental Hollow used by Sōsuke Aizen years before the start of the series. In his natural state, he exists as a six-limbed creature with a large, flame-patterned mask and a flock of tentacles on his back. When a Shinigami grabs these tentacles, their Zanpakutō is destroyed; however, the power is only good once every night. Metastacia's main power is the ability to merge with Shinigami, taking over their body to use as his own. Unlike human possession (two souls in one body), the fusing of two spirits is irreversible. After a short period, he is able to eat them from the inside-out to re-create his original form.

Metastacia took over the body of Miyako, a Shinigami and Kaien Shiba's wife, and used her to kill her fellow squad members before eating her. When Kaien Shiba battled Metastacia, he too was possessed and Rukia Kuchiki was forced to kill him to destroy the Hollow. However, Metastacia was designed to return to Hueco Mundo after being killed, so he could be analyzed. Still merged with Kaien at the time, this led to the two becoming one being upon returning. Sometime after, the merged creature was eaten by Aaroniero Arruruerie, giving him not only Metastacia's abilities, but Kaien's as well.

Metastacia's name most likely originates from the Greek word metastasis, meaning 'a change of state', which also refers to the spread of cancer throughout the body.