Possession is the process whereby a person's body is taken over by a spiritual entity of some kind.
Note that in strict terms possession applies only to people - anything souless (animals, objects etc.) can only be "infested" rather than possessed (although exorcism may well still work). If an entity is merely hanging around being a nuisance then that is properly termed oppression instead. The occupational of a human body which is nonetheless souless for whatever reason is an interesting and much debated case.
If you happen to be a shaman or otherwise part of a spiritualist religion then possession may be a normal part of your religious practice, but otherwise the most common variant would be Demonic possession and everything may hinge on what you consider to be a demon. Possession may be voluntary or involuntary - and that defined by various levels of consent - but in general, the more benevolent the entity, the more and better the consent it will seek before taking over. Of course, if you happen to worship the being in question (or its superior) then your consent may be taken as read…
The level of possession may also vary, from a full takeover of the body that expells the previous resident, to an annoying squatter … telling an inactive possesor from a particularly persistent oppressor may take some work.
Motives for possession will also vary - from simple joyriding to instinctive behaviour (especially in the case of non-sapient spirits), to the desire to cause mayhem, to specific targetting of the possessee's soul for damnation. As already noted the possessor may also be invited in. In general we can divide possession into two broad categories: hostile and non-hostile.
This is the sort of possession practiced by demons, evil spirits and similar things and traditionally begins with the victim somehow coming to the notice of a possessing demon - any kind of occult activity has the potential to cause this to happen, but it can also result from innocent contact with infested places or things. The demon will then lay siege to the victim, awaiting an opening and likely indulging in oppression whilst it waits. A variety of things, including further spiritual impurity or surrender to despair can create the necessary breach, after which the demon takes control of the victim's body1. This process may take a significant length of time, depending on the victim's strength of will and the power and skill of the demon, but generally, sooner or later, the demon ends up in charge and uses the victim's body for whatever ends it finds amusing. Some of the more incompetent demons (and, more often, spirits) are incapable of successfully operating a host body and are likely to leave it catatonic when in active control2 … others may negligently or accidentally cause harm to their host by trying to operate the body beyond it's safe limits, making it do things that it wasn't designed to do, using it to channel too much supernatural energy or just forgetting to make it eat or drink.
Once in control, a possessing demon is likely to get up to all sorts of things - none of them good - contingent on its personality. These could vary from turning the host into a great ruler to making him eat his own fingers… making life miserable for anyone in the hosts immediate area is typical, whatever the demon's long term plans are.
In some traditions a possessing demon can physically transform the host into a shape it finds more pleasing - this is suggested as a possible cause for lycanthropy. Demons in general are not good at looking after their hosts, and the body will generally deteriorate over time until it eventually dies3. The soul of a possessee is not necessarily damned, although the experience of being possessed is liable to cause a loss of faith for a variety of reasons, and so they should be considered at very great risk of being lost.
The expulsion of a possessing (or infesting) demon from its host is known as exorcism, whilst the disposal of an oppressing demon is known as deliverance.
Diagnosis of possession can be problematic as the symptoms can often be mistaken for a variety of mental disorders - the possessed are likely to demonstrate a fear of or revulsion towards sacred objects and religious people, but this sort of behaviour is relatively easily rationalised away by psychiatrists and in any case can often occur in genuine mental illnesses as well. Also, all but the most incompetent demons are generally supposed to be able to submerge themselves and hide within the host to avoid detection - although some may just be very good at roleplaying their host.
Some cultures also hold with the idea of demons or spirits that embody a certain disease and, on possessing someone (generally in an otherwise inactive, squatting sort of a way), inflict that disease in a way that doesn't correspond properly to the natural process and either resists treatment completely or recurs until the root cause is dealt with.
"Semi-hostile" possession may also occur to bodies left untenanted (due to astral projection and similar things) or where the resident soul is only semi-resident - perhaps due to being very close to death. These spirits needn't necessarily be altogether evil, but they are likely to be selfish and opportunistic at best.
In this case, possession proceeds with the (hopefully more or less informed) consent of the possessee (at some level or the other), hopefully for a pre-agreed duration and under pre-agreed conditions. The entity moves in, does its business and then leaves - the outcome of the possession being determined beforehand.
Motives for this sort of possession might include repaying the spirit for a service it has already done for you, or using its powers and abilities through you for some purpose. In some cases, a more powerful, benevolent spirit might be called upon to evict a hostile possessor.
Some traditions also speak of "walk-ins" benevolent spirits taking over bodies abandoned by their resident souls (usually as a result of complete despair or other irretrievably mental breakdowns), allowing the spirit to achieve some positive goal in the material world. Telling this from a "semi-hostile" possession (as above) may be tricky.
Game and Story Use
- A staple of supernatural horror campaigns, though preferably this should only happen to NPCs unless the PC in question does something really stupid, as most players will resent losing control of their characters.
- Though if you have a willing and convincing role-player in your group, you could let him play the demonic entity and see how long it takes the other characters to figure it out.
- You could start the campaign with one of the PCs as demonically possessed.
- It could even be an integral part of the character concept - such as Greg Keyes' Fool Wolf, or, more prosaically, a voodoo bokor who is frequently or continually ridden by one of the darker loas.
- The "demon" could actually be the PC, allowing him the interesting (but squicky) possibility of changing bodies when he gets bored with the current one (or when it dies). Potentially a very good way of infiltrating somewhere…
- Note the possibility of possession as a result of dicking about with (black) magic … or doing other morally dodgy things, especially in a bad place.
- For a system that uses critical failures in the magic rules, this might well be a good one: either becoming a victim of demonic oppression or effectively summoning a demon inside yourself.
- Inflicting possession on someone else is an interesting subtrope - often done either to please the demon by giving it a physical body, or as a vessel in which it will then be imprisoned and forced to serve the binder. Traditionally this requires breaking the will of the possessee in advance, usually by psychological torture, to make it easy for the demon to move in.
- Examples of possible applications include using the demon's magical powers and occult knowledge or using it to weaponise the host.
- The whole possession vs. madness is an interesting dynamic, especially once compulsory treatment kicks in.
- Some traditions also allow possession by benevolent entities - generally with either consent or vacant tenure.
- A walk-in might be an interesting background for a PCs.
- In a sci-fi setting, some form of possession may be available to AIs, with or without the overwriting of the host's personality.