A ghost is a malevolent, evil spirit whose presence drains years from the lives of those who encounter them, inching them closer to the grave. It is also known to possess its victims via an effect similar to magic jar.
Ghosts are often motivated by a specific agenda that allows them to cling to unlife, often related to the pursuit of vengeance or punishment of an individual. Jilted lovers or betrayed friends or victims of cruelty may become evil in their desire to punish those that made them suffer, and cling to unlife until their need for ruthless justice is sated, even long after such a thing can be reasonably accomplished. One specific kind of cruelty that often results in a ghost is a failure to show proper respect for someone's remains -- a ghost may animate near the location of its body, becoming resentful and hateful of the beings who still live. Those who were monstrously evil in life or whose deaths were particularly emotional, may also leave an echo of their existence as a ghost.
Ghosts haunt particular places relevant to their agendas, and this may or may not be the same location they died in.
Ghosts are largely solitary, but in rare circumstances, entire groups of people can become ghosts at once. A famous example would be ghost ships, ethereal vessels which sail the seas crewed by ghosts, and encountered in the presence of St. Elmo's Fire manifesting within a ship's rigging.
Ghosts may be laid to rest by satiating its desires, such as by burying its remains or extracting vengeance on its behalf. A ghost's needs can be quite demanding, however -- many heinous crimes have been comitted by those trying to lay to rest these hateful undead.
A ghost is active in dark places, or at night. They are largely naturally ethereal, though they are not naturally invisible - creatures on the ethereal plane or the material plane can see them quite clearly (though on the material plane, they are translucent). The mere sight of a ghost, however, conveys its horrible supernatural energies -- those who see a ghost must save vs. spell or be aged 10 years and flee in panic from the ghost for 2d6 turns. Priests above 6th level are immune to this effect, and others above 8th level have a +2 bonus to their saves against it.
Those that do not flee confront a highly intelligent (Int 13-14) creature of Lawful Evil alignment. Ghosts are normally immune to all attacks and spells. Creatures within 60 yards of a ghost are subject to a special kind of magic jar attack -- the ghost requires no receptacle, and if the target's save vs. spell fails, the ghost simply inhabits its body, suppressing the mind and spirit that is there for the duration. If the ghost cannot possess their target with this attack, their next approach is to kill the creature. The ghost must manifest on the material plane to strike, rendering it subject to attacks by silver weapons (which do half damage), holy water, or magical weapons. In its materialized state, it has an AC of 0. If they hit their target (THAC0 11), the victim is aged 1d4*10 years. A victim killed by a ghost's attack can never be raised from the dead -- this powerful supernatural aging is death itself. Ghosts move at a movement rate of 9, and, being ethereal, can move through solid objects (even when manifested).
A creature can become ethereal to attack a ghost on their own plane. In this state, they can be hit with normal weapons or spells (AC 8), and a cleric of 7th level or higher is capable of turning a ghost.
As an insubstantial creature who remains in the material world due to their desire to accomplish a goal, ghosts may be considered a type of geist.