Metastacia is an experimental Hollow created by Sōsuke Aizen that exists naturally as a six-limbed creature with a flame-patterned mask and a cluster of tentacles sprouting from its back. Its most distinctive ability is possession—when it fuses with a Shinigami, the merger is permanent and irreversible, unlike normal human possession where two souls coexist. Given enough time inside a host body, Metastacia can consume them from the inside out and regenerate its original form. It also has a secondary power: grabbing its tentacles destroys a Shinigami's Zanpakutō, though this only works once per night.
Metastacia's most infamously cruel act was possessing Miyako, the wife of captain-class Shinigami Kaien Shiba, and forcing her to slaughter her own squad members before consuming her. When Kaien confronted the Hollow, he was possessed in turn. Rukia Kuchiki was forced to kill him to stop the creature—but here's where Aizen's design gets twisted. Metastacia was engineered to return to Hueco Mundo after death so it could be studied, meaning it retreated while still merged with Kaien's body. The two fused completely upon arrival in the afterlife.
That merged form didn't last long. Aaroniero Arruruerie devoured the creature's spirit body, inheriting not only Metastacia's possession abilities but also Kaien's strength and techniques—a fusion that would haunt the living world years later.
Content compiled by AnimeList.moe from publicly available sources.
