These could be the most powerful of undead, save the Nazgûl. They were total energy, having no physical component at all, and were activated solely by the presence of living beings of the same type that they once were (most often Men). They appeared as they remembered themselves being, with varying degrees of effectiveness: lesser Ghosts often neglected pertinent details, such as appropiate clothing for the occasion. They also did not realize that, since their appearence was nothing but illusion, their will could affect it. A Ghost who wished to be larger and more menacing was so, with nothing but a thought. Greater Ghosts, whose instincts were under more control, avoided this giveaway and behaved in a very lifelike manner, enabling them to drain the constitution (and hence lite- energy) of a victim while engaging him in conversation or merely being near him.
It was a strange fact that most Ghosts seemed not to realize that they were dead; often, if they could be convinced of their true nature, they would dissipate and go forth to their appointed rest. (Note that Elves did not have Ghosts, since their spirits always knew where to go. It was notable that Elves did not fear Ghosts, perhaps for this reason). However, Ghosts were remarkably impervious to reason on this score (How would you. Reader, react if someone insisted that you were dead?), and would-be persuaders were well-warned that there was no way that the Ghost could avoid draining their life-energy, any more than they themselves could avoid having their hearts beat. Anyone near enough to a Ghost to speak to it was near enough to be drained.
Some Ghosts were bound to a place or object that was very important to them in life or that figured strongly in their deaths: a sentry killed while guarding a crucial post, for instance, whose spirit remained "on duty" long after the fortress that he guarded had fallen to ruins; the murder victim haunting the gold that she was killed for; the child plague victim still holding fast to the wooden doll that comforted its last hours.