Possession is a Dark Magical effect of one person’s spirit inhabiting or taking over the body of another.
The only examples of possession are the actions of Voldemort, the greatest Dark wizard of the age.
References from the canon
- Voldemort possessed Quirrell in order to monitor and control him. In this particular case, Voldemort's face appeared protruding from the back of Quirrell's head (PS17).
- He used this horrible form of intrusion on snakes and small animals while hiding out in the wilds of Albania without a body of his own. (GF33)
- Tom Riddle's memory form, freed from its diary by life energy from Ginny Weasley, inhabited the little girl off and on, forcing her to kill roosters, write on the wall, and so on. She talked about it later and said that there were long periods of time when she couldn't remember what had happened to her (OP23).
- During the climactic battle between Dumbledore and Voldemort, the Dark Lord possessed Harry, trying to make Dumbledore have to choose to kill Harry in order to defeat Voldemort. Harry remembered the love of his mother and friends and the feelings of love drove Voldemort out of him (OP36).
- While a Horcrux is still intact, the bit of soul inside it can flit in and out of someone if they grow too close emotionally to the object acting as a magical container (DH6).
The idea of Quirrell having Voldemort's face on the back of his head may have been inspired by Edward Mordake, a figure of late nineteenth-century urban legend who had a menacing "demon face" on the back of his head (source: Wikipedia). -BB