Summary
Metastacia is an experimental Hollow created by Sōsuke Aizen prior to the series' beginning. In its natural form, it appears as a six-limbed entity featuring a large mask with flame patterns and a cluster of tentacles on its back. It possesses the unique ability to fuse with Shinigami, permanently taking over their bodies without the possibility of separation. Once merged, Metastacia consumes its host from the inside to regenerate its original shape. It can also destroy a Shinigami's Zanpakutō if the blade touches its tentacles, though this specific power activates only once per night. The Hollow first possessed Miyako, the wife of Kaien Shiba, forcing her to kill her fellow squad members before consuming her. When Kaien engaged Metastacia, he too was possessed, necessitating that Rukia Kuchiki kill him to eliminate the threat. Because Metastacia was designed to return to Hueco Mundo after death, its merger with Kaien resulted in the two souls combining into a single being. Upon their arrival, the merged spirit was consumed by Aaroniero Arruruerie, granting the Arrancar both Metastacia’s and Kaien’s powers.
