First of all, lets talk about possession.
Who is vulnerable to possession?
It is commonly thought that Wiccan, pagans and other people who practice the occult are more susceptible to possession because they are more receptive to otherworldly phenomenon, and also because they may deliberately invoke certain spirits. However, this is only partially true. Novices or people who are merely dabbling in the occult may be more susceptible to possession, as they may not follow safety procedures such as drawing a circle properly. Experienced witches and occultists usually possess a lot of will power and are highly resistant to possession.
Apart from novice occultists, others who are vulnerable to possession are people who play with tools such as Ouija boards, or who visit places which are crowded with spirits (for example old cemeteries and haunted houses). Being in the wrong place at the wrong time can be even worse if these people are naturally susceptible to possession this could be because they are young, mentally unstable, or have undeveloped and untrained psychic powers.
How Possession Works
Possession can manifest itself in a manner of ways. In theory, any conscious entity can possess a human, but the most common are demons, followed by the ghosts of humans.
Demons usually possess people either to hurt them directly (inciting risky, borderline suicidal behaviors) or to attack other people. A demon may have problems controlling their victim entirely, depending on the victim, and also on the strength of the demon. But the longer a demon stays in the body of a possessed person, the more detrimental it will be, as it will try to break the bond between the soul of the person and their body, which can lead to death.
Usually, a demon will not leave the persons body unless they are forced to, or unless they reach their goal and no longer need the physical body.
Malicious ghosts that have a grudge against the living will sometimes use possession to achieve their purposes. As ghosts have lived in human bodies before, they are better at controlling these, but their energy is less than most demons, therefore possessions tend to be intermittent and very short, and affect only the most vulnerable people (for example children). Often, the ghost will leave the body of their victim after only a few minutes, exhausted by fighting against the Will of the person.
Symptoms of Possession
Although rare, possession is a phenomenon which is very frightening to witness. If the victim does not have the internal strength to fight against the invader, they can lose consciousness completely, and have no memory of the episode when the possession ceases. If the victim does struggle against the spirit, they retain their consciousness of the whole possession.
The main symptom of possession is observable behavioral changes. The victim may become violent towards others or themselves for no apparent reason. Sometimes inexplicable pain may assail them, and injuries may suddenly appear on their bodies.
Occasionally, there may be signs so obvious that there can be no doubts about what is happening. The victim may demonstrate superhuman strength, or even use supernatural powers such as telekinesis (moving objects with thought alone) or pyro-genesis (setting fire to objects with thought alone). Some demons may change the look of the possessed person a common sign is completely black eyes.