The clearest evidence we have that demons exist comes from accounts of violent “poltergeist” hauntings and exorcisms carried out by recognized religious authorities. Remember that demons are primordial beings and (at least theoretically) have never lived a human life. They are the embodiment of evil.
Demons are inhuman spirits, or at least very ancient and evil human souls whose history throughout long eons of time have taught them how to manipulate huge amounts of spiritual energy for exclusively evil purposes. Their hauntings are different than regular ghost hauntings. A ghost is an “ordinary” earthbound human soul. It is usually a passive entity with limited powers and abilities. Ghosts behave in predictable ways, either wanting to communicate their plight or be left alone to contemplate their own misfortunes. They will materialize, sometimes communicating and sometimes not, and then disappear. If a ghost causes psychokinetic effects, they are minor, like the levitation of a figurine, the throwing of a small stone the turning of a doorknob or the breaking of a teacup. They can also make noises, (sometimes loud noises), or move small objects from place to place. According to Ed Warren, ghosts rarely levitate anything that weighs more than a pound. They can be scary, but their manifestations are relatively weak, and if the entity isn’t malicious, a family can even coexist with a ghost in the house. Most ghosts are indifferent to religion and religious icons.
Demons, on the other hand, are truly malevolent. They cause tremendous psychokinetic damage, sometimes leaving an entire house in shambles. The phenomena they produce are much more powerful and frightening than any ghost can produce. They can levitate heavy objects like refrigerators, washing machines and even people. In some exorcisms, large objects like chairs have even been seen to explode apart! If a demonic entity actually materializes, it often appears as a black whirling mass, which is their actual appearance. They most often present as black shadow figures so dark that people describe them as blacker than black, however, demons and devils can temporarily assume virtually any form they wish. Like any spirit, its outward appearance is created by its own inner thought processes. They also are accompanied by a strong stench of sulfur, excrement, rotting flesh and ozone.
The devils in the upper hierarchy are infinitely intelligent and infinitely knowledgeable. Theoretically, they have existed since the beginning of time and have witnessed the entire history of mankind. The demons in the ranks are not as intelligent, and some are downright stupid, but they still have a lot of knowledge because they too have existed since the beginning of time. They gain much of their knowledge from the living souls that they have oppressed over the centuries. They have the ability to read the minds of any living person they come into contact with. No living person can ever outwit or overwhelm a demon. Only a divine being can do that.
Note: In a way, the fact that these things exist and make their existence obvious is a sort of backhanded confirmation that God exists. If God didn’t exist, the demons would have no reason to hate him so much. God never really reveals himself to us as a material or even a spectral being. He works exclusively in the background, like His angels, and it is our job to find Him, and not the other way around. The demons’ incredibly blatant displays of hatred for God are simply an indication of their deep belief in Him.
Fear is the psychic currency of demons. Demons need fear on the part of living persons as a source of energy in order to manifest. Fear in a person causes a constant leakage of psychic energy into the spiritual environment, and the demons feed on that. That’s why they cause the havoc they do in people’s lives. The more fear, hatred and distrust they can provoke, the more energy they get. The more energy they get, the more power they have. Their strategy is to create fear in order to break down the human will. Their prime objective is to cause a momentary lapse in a living person’s free will. This creates an opening for possession.
Demons hate light. They perform most of their evil at night between the hours of 9:00 PM and 6:00 AM, and shining a bright light on them can sometimes stun them. Darkness is symbolic of the evil they love. They eventually try to overwhelm the five senses with evil phenomena such as disgusting smells, ghoulish moans, blood curdling screams, knockings, poundings, disembodied footsteps, rapid changes in room temperature, ghastly visions, heavy breathing and unintelligible whisperings.
Demons lie about everything and can often fool a medium into thinking it is only an “innocent” ghost, like a lost child. This is one of the ways they can gain permission to enter a home or attach themselves to household objects, ultimately aiming to possess the body of a vulnerable human being.
Demon infested houses can sometimes be cleared by a knowledgeable psychic if the infestation hasn’t taken a very firm hold on the property. They might do this using rituals such as blessings, Kosher salt applications and smudging (the use of burning sage). Properly applied, this may work in all stages of a demon haunting up to and including the stage of infestation. This is NOT the case with a demon oppressed or possessed person. The only way to remove a demon who has infected or stolen a human body is through religious exorcism. Psychics and mediums are not suitable instruments in the case of demon possession.
The only thing that can (sometimes) free a human soul from demon possession is a holy person with a serious belief in God and an exorcism ritual. Because demons are evil angels and not the spirits of evil men and women, they cannot be removed from a person’s body by any human agency without the intervention of the angels working for God. That is the reason that successful demon exorcisms are never done in the name of the exorcist himself, but in the name of God or Jesus (or in the name of an avatar that is worshiped by the people of other religions).
If you’d like to learn more about demons, devils and exorcism, you can start with the following book: The Demonologist: The Extraordinary Career of Ed and Lorraine Warren, written by Gerald Brittle, 2013