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Metastacia was an experimental Hollow created by Sosuke Aizen when he was trying to perfect the process of Hollowfication. Metastacia is responsible for the murder of Kaien Shiba and his wife Miyako.
Appearance Metastacia is a rather large green-skinned, six-legged monstrosity with a flock of thick, maroon-colored tentacles growing out of his back and wears a large, flame-patterned white mask with two large circular eye-holes and four backwards-curved blunt-ended "tusks" which line both sides of the mask, two for each side.
History Metastacia was born as the result of one of Sosuke Aizen's many attempts to perfect Hollowfication but, as Aizen would later comment, was a failure. When Metastacia was unleashed Miyako, the wife of Kaien Shiba, and four other Shinigami were sent out on a reconnaissance mission to learn more about it, as Metastacia was unlike any Hollow The Soul Society had encountered before.
Metastacia killed all members of the reconnaissance team with the exception of Miyako, whom he impregnated with himself and took control of her mind, and used her to slaughter even more of her fellow squad members before ultimately eating Miyako from the inside-out, leaving only the tattered remains of her standard issue Shinigami Shihakushō laying on a tree branch as evidence of her death for her husband Kaien along with Rukia Kuchiki and Captain Jūshirō Ukitake to find.
Swearing revenge on Metastacia, Kaien asked that he be given permission to face Metastacia alone, which Ukitake granted without hesitation. When Metastacia revealed himself and engaged Kaien in battle following a short bit of banter, Metastacia showed Kaien just how unmatched he was in terms of power by dissolving his Zanpakutō with merely a touch.
Despite this, Kaien continued to engage Metastacia in battle only to end up being fused with him. With no other choice Captain Ukitake stepped into the fray before Metastacia could kill Rukia, whom he had chosen as his next victim, and told her to run while he held Metastacia back.
When Ukitake said that he swore to free Kaien from Metastacia's influence, Metastacia laughed and began to explain to Ukitake that he had not merely possessed Kaien but had actually fused with him. Also, because they were both spirits any attempt to separate them would be a futile effort as the merger of two souls was an irreversible process and nothing at all like possession.
Once realizing that Kaien was truly dead, Ukitake stopped holding back but before he could engage Metastacia any further in battle he was stricken by his illness, leaving Rukia, who had stayed behind despite the order to run that Ukitake had given her, completely defenseless as Metastacia turned his attention back to Rukia and leaped at her.
As Metastacia descended upon her, Ukitake told Rukia that Kaien was no more and that she had to kill Metastacia before he could wreak further havoc upon The Soul Society. Rukia did as Ukitake commanded and stabbed Metastacia in the heart with her Zanpakutō.
After his death, Metastacia's body returned to Hueco Mundo just as it was designed to do, but because he had been fused with Kaien at the time his corpse ended up accompanying Metastacia's back to the world of The Hollows. Upon materializing, the corpses of Metastacia and Kaien merged back together.
Sometime later the corpse of the merged entity that had once been Metastacia and Kaien Shiba was consumed by Aaroniero Arruruerie, which not only gave him Metastacia's powers and abilities but Kaien's as well.
Powers and Abilities Unlike other Hollows, Metastacia was the result of an experiment conducted by Sosuke Aizen and therefore possessed unique abilities which other Hollows did not.
Body Possession Metastacia is capable of possessing anyone of his choosing by merging his soul with that of his victims which he does by dislodging the flock of tentacles on his back and has them enter the chosen victims body. Once Metastacia has taken control of his victims body their skin becomes green and their faces begin to partially resemble his mask. While it is presumed that this technique was passed on to Aaroniero Arruruerie, he was never seen using it.
Zanpakutō Destruction While this technique works only once a night, Metastacia can instantly destroy a Zanpakutō if its wielder grabs a hold of any one of the tentacles on his back. Like Body Possession, it is presumed that this technique was passed on to Aaroniero Arruruerie but was never seen being used by him.