Metastacia

In his true shape Metastacia is a six-limbed beast crowned by a broad, flame-marked mask, with a mass of tentacles trailing from his back. When he merges with a host, that body warps to mirror him, taking on his blackened eyes, the fiery pattern ringing them, and a sickly grey-green skin tone.

A genuinely vicious Hollow, Metastacia relishes devouring Shinigami. He claims to feel regret only in the sense that he wishes he had eaten more, lamenting that he never devoured Miyako Shiba's torso or head. Sadism defines him: he delights in mocking Kaien and Rukia and cruelly maneuvers her toward striking down her own comrade.

In the years before he abandoned Soul Society, Aizen labored to reshape Hollows into something close to Shinigami, and Metastacia stands among his successes. The creature wipes out the reconnaissance squad of Miyako Shiba, prompting Kaien Shiba, Rukia Kuchiki, and Jushiro Ukitake to give chase. Two abilities make Metastacia deadly: brushing the tentacles on his back destroys a Shinigami's Zanpakuto outright, a trick usable once a night, while his Spirit Body Fusion lets him fire those tentacles into a victim and seize control of the body as his own crumbles away. When Kaien makes contact, his blade breaks, and Metastacia then overtakes him from within, turning Kaien against Rukia. Ukitake shields her, but his illness flares, and it falls to Rukia to run the fused pair through, ending both Kaien and the Hollow. True to Aizen's design, Metastacia returns to Hueco Mundo with Kaien still bound to him, where Aaroniero Arruruerie devours him and absorbs the powers of both.