To be technically correct, what you are might be thinking about are the unbirths. The American pop. believed that this is what it was called until it was proven that unbirth was a translation error, and is ACTUALLY called an unvexed. That's all I know. For some details on things like origin, and how they're created, ask some1 else.
Well, my best guess is that it's the third piece of a person. For those who don't know, keep reading. The heart, the body and the spirit. The heart becomes a Heartless, the body becomes a Nobody(and by my best guess), the spirit becomes a Birthless. Nobodies need spirits to walk and once extinguished passes onto a Birthless form
These little guys are actually the heartless & nobodies. I'll go into detail when people died the became unbirths & thanks to Xehanort Braig Ezio and the other 6 apprentices reaserch into the heart when people died the split into nobodies & heartless
