The haunt is an undead soul motivated by intense emotion to complete some task that it died before completing. They do this by possessing the bodies of the living. Haunts are near-automatons, mindless creatures compelled to complete their final task at any cost, and most have no ability to communicate what that task is or how it might be accomplished. They simply possess...and move forward...
A haunt is born when a creature dies in the process of, but before, completing some task. The motive for the task is always emotionally powerful, and involve concepts such as love, hate, revenge, and malice. A haunt is linked to the site where it fell, and cannot move more than 60 yards from where they died without possessing a living body. This region is called the haunt's domain.
A haunt typically resembles a translucent image of their former body, often at the moment of death. A haunt can also transform into a luminescent ball of light, however looking something like a will-o-wisp. In either form, a haunt seeks out visitors to its domain and mindlessly attacks them, hoping to posesses their body. A haunt is mindless (0 Intelligence), but quite persistent (Champion (16) Morale). It has the same alignment it had in life.
In its natural state, only silver or magical weapons, or fire, hurt a haunt. Weapons only inflict 1 point of damage (plus any magical bonus), and normal fire inflicts 1 hit point of damage each round. Magical fire, however, inflicts full damage. In their natural state, haunts have an AC of 0, and 5 HD. Because they are so tightly bound to their domain, a haunt cannot be turned. If destroyed, a haunt fades away, but reforms 1 week later to haunt the same locaiton again until its task is complete.
A haunt that attacks a creature does so with a THAC0 of 15, and, on a hit, it reduces the target's Dexterity by 2. A creature reduced to 0 Dexterity becomes host to the haunt that reduced it. Lost Dexterity returns at the rate of of 1/turn.
If the alingment of the haunt is morally opposite that of its new host (a good haunt inhabiting an evil creature, or vice versa), it seeks to end the host's life by strangling them with their own hands. On the first round, the host suffers 1 point of damage, and this doubles every round until the host dies or the haunt is forced out of the body.
If the haunt and the host are not moral antagonists, the haunt uses the body to continue its mission. It has no concern for its host's form, and it does not eat or sleep. While possessing a creature, the haunt cannot use equipment or abilities that rely on special knowledge (such as spells), but can employ any physical gear the character has just as the character themselves would. Attacks directed at a possessed character simply hit the possessed character's body and deal damage to them. The haunt defends its body as best it is able to. If its host dies, the haunt's domain is considered 60 yards from this new location, rather than its old one, so a haunt may make gradual progress over years by linking a series of unfortunate bodies together.
If the haunt manages to complete its task, it passes onto its final rest and departs the body (leaving it with a Dexterity of 3).
A haunt can be safely extracted from its host in two ways. In the first, if hold person is cast on the host, the haunt must make a save vs. paralyztion or be ejected from the body. In the second, if dispel evil or dispel good are cast on the haunt's host, the haunt will be destroyed forever as appropriate to its alignment (dispel evil slays evil haunts; dispel good slays good ones).
Haunts are powerful, and managing to lay one to rest is worth 2,000 XP (a Level VII challenge).
Some 10% of haunts retain some knowledge of their former life, and have an Intelligence similar to what they had in life. These haunts can be communicated with, though they still feel the overwhelming drive to complete their task, and they will possess the living to do so. They are often remorseful at having to do this, but they have little choice -- the cling to reality by force of will alone.
A haunt may be considered a type of geist.