 As one of Sōsuke Aizen's creations, Metastacia specializes in hunting Soul Reapers as his tentacles have the ability to shatter zanpakutō while he can merge into the body of a Soul Reaper, possessing the host permanently.When his lair in Soul Society is attacked by the Soul Reapers, Metastacia possesses Kaien Shiba after having assimilated his wife prior, forcing Rukia Kuchiki to defeat him by impaling Kaien's body.He is at that point believed dead, but it is later revealed that Metastacia survives long enough to return to Hueco Mundo, where Aaroniero Arruruerie devours him and indirectly gains Kaien's powers.