Metastacia during his fight with Kaien Shiba
Volume 16, Chapter 134
Metastacia (Metasutashia) is an experimental Hollow created by Sōsuke Aizen years before the onset of the Bleach main story. In it's hollow state it is a six limbed creature (with the posture similar to that of a spider) with a flame patterned hollow mask with a mass of tentacles with protruding from from his back. When a shinigami is to grab one of Metastacia's tentacles then his/her zanpakutō is destroyed, but because of the phenomenal power, it can only be used once every night. However, Metastacia's main power is having the ability to merge with a shinigami's body and use it to suit his purpose. After fusing with the body the process is then irreversible (unlike human possession in which 2 souls exist in one body). The only way, then, for Metastacia to revert to his original form is to eat his host from the inside-out.Rukia Kuchiki, we see Kaien Shiba fighting with Metastacia. However, Metastacia took over the body of Kaien's wife - Miyako Shiba, and used her body to kill several of her squad members before eating her. During the battle between Kaien and Metastacia he too was possessed by him and Rukia was forced to destroy both Kaien and the hollow through stabbing Kaien's body. But Aizen had designed Metastacia to return to Hueco Mundo after being killed for examination but due to the hollow still being merged with Kaien when he died, the 2 returned to Heuco Mundo as one being. But upon returning back, the Espada member Aaroniero Arruruerie consumed the body which resulted in him gaining both Metastacia and Kaien's powers.
Metastacia's name could have derived from the Greek word "Metastasis" which means "A change of state", it could also refer to the spread of cancer throughout the body.
 Powers & Abilities
Because Metastacia was an experimental Hollow created by Sōsuke Aizen, he possesses unique abilities.
- Body Possession: Metastacia has the power to merge with any body he chooses by transferring his spirit body with theirs. This ability is shown explicitly through his battle with Kaien Shiba, the 13th Division lieutenant, in which it was also theorized that whenever he possesses a body, parts of his hollow mask also show through on the face. Presumably this ability was also passed onto Aaroniero but never used it, most likely due to the fact the process was irrevocable.
- Zanpakutō Destruction: If a shinigami were to grab one of Metastacia's tentacles, their zanpakutō is destroyed. This power is limited to be used only once every night and is the only reason he defeated Kaien easily and in little time. Again, this would've been passed into Aaroniero, but was never shown.