An abstract of the bibles secret gospel of Judas:

Lingering Souls, Wandering Souls or Ghosts are all Names given to souls that linger in the world of the living. Those are souls that either can't move on or that don't want to move on. The ones that can't move on are pretty harmless. The dangerous ones are the ones that have a strong will to stay in the world of the Living.

What makes the difference between the two kinds is, that the first one never came in contact with the underworld. The restless kind is comparable with a Fish on the hook. No matter how hard they want to move on there are external factors holding them back. For them to move on requires someone or something to remove the hook. Sufficient for this can even be the passage of time and the fall, of the capturing reason, into oblivion. Most of the times, the lost souls are a total replica of the human they were before they died. Of course there are evil personalities in some living humans too but those are the rare ones. A second reason why lost souls are not as dangerous is because they have no physical or psychic power. They are bound by the reason they can't move on and have a hard time even just communicating with the world of the living.

The more dangerous kind are those souls that choose willingly to escape the Underworld. It is not as easy as it sounds to escape the Underworld that is guarded by the old ferryman and Kerberos. Usually souls go through the last judgment when they die and arrive at the Underworld. The arrival is not optional, it is more of a natural reaction of the World to dying and nobody can resist it. The last judgment decides what level of the Underworld is your eternal fate. While the first levels are comfortable to be in and are seldom escaped from, the deepest levels are the embodiment of what people in the World of the living call Hell. Souls that come here are evil by nature additional to that the deeper levels can make even the most rational minds go crazy. One will suffer eternal pain of much higher intensity than it would ever be possible to feel pain as a living being. The mind gets invaded by the evil aura of emotions caused by the endless torturing of evil minds. A soul that experiences the deep levels slowly transforms into Daemons. The more Souls a Daemon can refine into himself the stronger he gets. So there are lower Daemons like yokai and succubus or other lower devils that torture one soul at a time. They use methods like burning the souls, drowning them or abusing specific personal fears in realistic illusions and nightmares. The variety of methods goes from being eaten out by bugs to imprisonment in Silent darkness for thousands of years. Apart from those there are even stronger ones. The stronger a Daemon gets the more eccentric get his Methods. That is caused by the fusion of a large amount of souls. The result is a paradox personality. Often times this leads to madness. The worst are the Arch daemons like Behemoth, Lucifer, Beelzebub, Satan, Mephistopheles or Dagon. They have collected so many souls over the years, that they are driven by the lunatic desire to collect souls. A soul captured by those powerful psychopaths is tortured in ways not comprehensible by human beings. Some forcefully combine human souls and animal souls. Others are reviving your sanity every time just to see in what kind of craziness your falling next. Some of the worst ones are making you do the job of driving your loved ones to madness.

Usually the world of the living is not opened for those deeper level creatures. Nevertheless there are every once a while some that escape their eternal shackles and trick the old ferryman and Kerberos into opening the Gates to the Underworld. Living of a human host they roam around and collect their souls in the world of the living.