Ghosts were the souls of a deceased sentient creature that had been bound to haunt a specific area, object, or creature that held significance to them when they were alive. They could be trapped in this state for a number of reasons, such as having been horrendously evil in life, being denied a proper burial, or because they had unfinished business cut short by an emotional death. If that reason was resolved, the ghost would be able to pass on.
Being a form of Undead, ghosts had no need for the air, food, drink, or sleep that they formerly required in life. They moved about by flying; having incorporeal bodies, they could pass through solid objects and other creatures. Ghosts could travel to the Ethereal Border plane at will, could possess bodies of the living, and could instill magical fear into other creatures. They were able to move incorporeally through solid objects, and were able to wither life with a single touch.
Enhancedss or Spellcraft that disrupted undead entities were often necessary to destroy their trapped essence.