Sometimes a soul doesn't move on after death. This is usually because they are stuck in a pattern where they can't let go. It might be the way they died, waiting for a loved one, or any of several other things. They aren't always dangerous as some need to be heard or be guided onto where they should go. Priests of the Raven Queen are often looking for such souls. Ghosts, like the living, are varied in how they act. In some areas of the world, they stay mostly to convey a message to the living before moving on, in others, they are only vengeful spirits who attack on sight. What is always true is that the longer they have been around, the more they lose their original personality and just become an embodiment of the desires that keep them bound to the material plane. This, in turn, might make them dangerous as they let nothing come in the way of the patterns they follow. Typically, a ghost is bound either to a place, an object, or a creature. They won't stray far from what they haunt and it is central to their obsession. Destroying their anchor might free the spirit, destroy it. or just make it very angry. The latter is especially likely with ghosts held from moving on by strong emotions - positive or negative. The appearance of a ghost is that of a semi-transparent version of their living self, often desaturated and with a blue, red, or dark tint. They drift in and out of the Ethereal Plane, and sometimes the Shadowfell, making them invisible to others and able to pass through otherwise solid objects. They are normally still able to see into other planes from the near ethereal which means they can observe and then appear whenever or wherever they want.