Ningai

Ningai our spiritual beings that are cut out from each group because they are basically all of the types of spiritual being in one soul. Because they're so many things in one they can't die because they just keep coming back. They're Shinigami, Arrancars, Vaizards, and everything else rolled into one. It's exteremily difficult to kill a ningai becasue they can do whatever it takes to win with no trouble at all. Less experienced ningai tend to die but they don't know how to come back so they stay dead. Yamamoto hated the idea of ningai so he ordered them all to die. The problem is that they'd come back.