Possession is a form of magic which involves a person or a creature entering a person's body and taking complete control of it. This is very powerful form of magic and only very powerful and experienced sorcerers or creatures can achieve control over this terrible form of magic.
The only known person to practice this kind of magic was the very powerful sorcerer Cornelius Sigan. He gained immortality by transferring his soul into a jewel, and it would have possessed anyone who touched it. Sigan also had the power to kill people from inside as shown when he killed Cedric when leaving his body. When Sigan's soul tried to possess Merlin, he was able to trap it again into the jewel by casting a very powerful spell given to him by the Great Dragon (The Curse of Cornelius Sigan).
Goblins also have the power to possess humans. They can transform themselves into little gold spheres and enter a person's body. Once inside, they completely take control of it and they are able to use their magic powers too. The only way to force the Goblin out of the victim is to kill the person who's been possessed (Goblin's Gold).
The Sidhe can also implant a faerie inside an infant, and thus create a Changeling. In time, the faerie takes over the victim's mind and body. The Witches of Meredor, anyway, have developed a potion that can force the faerie out of the Changeling (The Changeling).
Another magical creature that is able to bend someone to its will (but not physically possess) is the manticore. The manticore can make its victim do whatever it wants, even not make them reveal that the person is under its influence. When this creature strengthens its control, the victim's eyes turn completely black (Love in the Time of Dragons). Similarly, the Fomorroh can also bend a person's will by planting an instruction into his/her head which becomes an obsession until it is complete (A Servant of Two Masters).
When Elyan disturbed a shrine, a spirit of a Druid boy began to control him so that he would have his revenge on Arthur for his death and so that he would find peace in the other world. Eventually the boy completely possessed Elyan to the point that Elyan spoke with the boy's voice and not his own. The boy was only released from Elyan when Arthur was filled with remorse for the boy's death as well as persecuting the Druids (A Herald of the New Age).