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.
The Lost Soul