A form of assault on humans an animals which is alleged to cause physical or mental illness or injury. Such attacks may be as the result of the action of a human sorcerer, or of malevolent spirits, or with the latter acting under the influence of the former. They may also be the result of action by a human during out-of-body experience. Symptoms experienced may include feeling a crushing weight accompanied by paralysis (the "hag syndrome"), feelings of overwhelming dread, the presence of bruises on the body after sleep, and poltergeist phenomena.
It has been estimated that hag syndrome is experienced by approximately 15% of the population worldwide.
The symptoms have been variously explained as due to sleep disorders or sexual repression.