Metastacia

Metastacia is an unusual Hollow that has the power to merge with Shinigami. He uses this power to infiltrate groups of Shinigami and kill them off one by one. He merged with Kaien Shiba when the latter tried to kill him for killing his wife Miyako Shiba, but he was killed by Rukia Kuchiki and later eaten by Espada Aaroniero Arruruerie. 

Powers and Abilities 

Metastacia is an experiment of Aizen's, hence its unique abilities.

Metastacia possessing Kaien

Body Possession: Metastacia could possess anyone he wanted to, by transferring his spirit body into their own. The only example explicitly shown was with Kaien Shiba; whenever he possesses a body, their skin turns green, eyes turn completely black, and orange splotches appear around their eyes, similar to the appearance of his Hollow mask.
Zanpakutō Destruction: If anyone touches the tentacles on Metastacia's back, their Zanpakutō will be destroyed instantly. This effect was limited to once a night, and was the only reason he had such an easy time against a Shinigami lieutenant.