Soul

This monster is born of the souls of humans who died with regrets, though the poor creatures no one remember how they died or what it was that tormented them so in death. They are driven purely out of resentment for living beings and they hold a particular envy for living humans. Many different methods of dealing with these souls have been attempted to try to reduce the large number of their victims. But given the impossibility of hoping that no one will ever again die with regrets, eradicating these monsters remains no more than a dream