Metastacia

Metastacia is malicious and methodical. Rather than relying only on physical violence, it exploits possession and the identities of its victims to inflict emotional harm.

It taunts opponents with intimate knowledge taken from absorbed souls and deliberately turns personal bonds into weaknesses.

Metastacia permanently fuses its spirit body with a Shinigami, seizes control and eventually consumes the host from within. Unlike temporary possession, the fusion cannot simply be reversed by separating two intact souls.

Its back tentacles destroy a Zanpakutō when touched, although this function can be used only once per night. Aizen also altered Metastacia so that its spirit would return to Hueco Mundo and reconstruct itself after apparent death.

Metastacia attacks members of the Thirteenth Division during a mission, killing Miyako Shiba and using her death to draw out Kaien Shiba. It fuses with Kaien, leaving Rukia Kuchiki to confront the body of her mentor.

Rukia impales the fused body, but Metastacia’s programmed return carries the absorbed Kaien back to Hueco Mundo. Aaroniero Arruruerie later consumes Metastacia, gaining access to Kaien’s appearance, memories, spiritual power and Zanpakutō. Metastacia does not independently become Aaroniero.

Sōsuke Aizen created and deployed Metastacia as an experiment. Miyako Shiba and Kaien Shiba are its principal victims.

Rukia Kuchiki destroys the body it shares with Kaien. Aaroniero Arruruerie later consumes Metastacia and inherits the souls and abilities contained within it.