Possession is the phenomenon in which an invading spirit takes root in the body of a mortal, often supplanting or outright excising the existing spirit or soul. Creatures capable of possession include ghosts and demons, though some magic uses like witches and warlocks may possess this ability as well. Forcing a spirit to leave a body usually requires someone with acute knowledge of the occult, a strong will and, if necessary, a strong standing with the Catholic Church. If the victim is possessed by a powerful demon, it is usually best to have an old priest and a young priest on hand ready to administer the Rites of Exorcism.
The Exorcist: In The Exorcist film series, a young girl named Regan MacNeil finds herself possessed by a demon named Pazuzu. She demonstrates tremendous supernatural feats including levitation, altering her surrounding temperatures, altering her voice, spinning her head 360 degrees and, of course, intense projectile vomiting. Having exhausted all conventional options at trying to cure her daughter, Chris MacNeil is forced to enlist the aid of an exorcist named Father Damian Karras. Karras in turn, brings in a more experienced pro named Lankaster Merrin - who had dealt with this particular demon once before. They succeeded in exorcising the offending spirit from Regan's body, but at the cost of both their lives.
Freddy vs. Jason: Occasionally, Freddy Krueger demonstrates the ability to take bodily possession of another. In one instance, he possessed a stoner named Bill Freeburg and used him to destroy supplies of a drug called Hypnocil. He then had him confront Jason Voorhees and inject him with two massive doses of tranquilizer. Jason cut Freeburg in half with his machete, but this did not affect Krueger.
Angel: Lonely Hearts: A Burrower demon named Talamour was hopping from body to body hoping to find a suitable host through which he could live forever. The bodies that he chose however, were unsuitable for his form and began rotting away. Whenever he was finished with a body, he would hop into a new host form, destroying the previous form in the process.