An experimental Hollow utilized by Sōsuke Aizen years prior to the beginning of the series. In his original form, he appears as a six-limbed creature adorned with a large, flame-patterned mask and a cluster of tentacles on his back. Should a Shinigami grasp Metastacia's tentacles, their Zanpakutō is shattered; however, this power can only be used once per night. Metastacia's primary ability is merging with Shinigami, overtaking their body and using it as his own. Unlike typical possession where two souls coexist in one body, the fusion of two spirits through Metastacia is irreversible. After a brief period, he consumes them from within to restore his original form.
Metastacia took control of Miyako, Kaien Shiba's wife, and used her to murder her fellow squad members before consuming her essence. When Kaien Shiba confronted Metastacia, he too became possessed, forcing Rukia Kuchiki to kill him to destroy the Hollow. However, Metastacia was designed to return to Hueco Mundo after being killed, allowing for analysis. At the time of his death, still merged with Kaien, this led to the creation of a single entity upon returning. After the merged creature's spirit body was consumed by Aaroniero Arruruerie, it granted him not only Metastacia's abilities but also Kaien's.
