Demonic Possession

Demonic possession is a skill used by many demons. They use it to possess and act with a human body. This has helped them protect themselves from those that hunt demons that are on Earth. Once possessed it is hard for the demon to be forced out of the human again. An exorcism can be preformed by a priest, witch, shaman or monk with various rituals. This is not always successful and can lead to the death of the host or the the exorcists. Once a possessed human is killed the demon is banished back to whathever hell dimension they were summoned from.
However, demonic possession can be resisted and or protected against. Magic users and other supernaturals are completely shielded against demonic possessions. Ordinary Humans do not have this kind of protection.
This is a very rare phenomenon as demons on Earth that can successfully possess a human are very rare. Most reported incidents are really just mental disorders or an attempt to force the possessed person into confinement.
Powerful witches can be possessed by demons when they lower their guard (either on purpose or by accident). Once they are possessed the demon and witch fight for control over the body. When the fight is won by the witch they can use the powers of the demon for their own gain. Of course always at the risk of losing control. This is considered Dark Magic by most and usually forbidden.