Seán Manchester (from his writings) on shadow beings:
While a number of witnesses believe that shadow beings act as benevolent guardians watching over us, just as many witnesses have no doubt they are demonic. Some believe shadow beings to be ghosts, but the many stories received and compiled convince others that these beings are a type of inter-dimensional phenomenon from which apparition is only one sub-category. Serious research into this paranormal (and possibly psychological) genre will paint a clearer understanding of the nature and make-up of these dark mysterious images. One thing you can be sure of is that the experience is real enough to those who encounter these strange apparitions.
Shadow-like creatures of modern folklore are attested by many witnesses who claim dark forms, seen mostly in peripheral vision, appearing like flitting phantoms in cloaks, hoods and sometimes hats. Reports of shadow beings are not dissimilar in many ways to sightings of ghosts.
Shadow beings are typically described as black humanoid silhouettes with no discernible mouths, noses or facial expressions, child-sized humanoids, or shapeless masses that sometimes change to human like form and featuring eyes that are either glowing or not apparent. Movement is said to be quick and disjointed, and some stories describe the visible outline of a cloak, and sometimes a wide-brimmed hat.
Images seen in peripheral areas of vision can be caused by pareidolia, a condition in which the brain incorrectly interprets random patterns of light/shadow or texture as being familiar patterns such as faces and human forms. The same condition can also be observed in macular vision in low light conditions, or when viewing a complex but random image. A common example would be perceiving a shadow, thrown by an item of furniture in a darkened room, as being a person. Hypnagogia, also known as "waking-sleep," a physiological condition in which a person is part-way between sleeping and waking, can also account for such perceptions. During hypnagogia, a person can be conscious and aware of their environment, but also in a dream-like state where they can perceive images from their subconscious. People experiencing waking-sleep commonly report the sensation of lights or shadows moving around them, as well as other visual hallucinations. A feeling of dread is also a sensation that occurs when experiencing hypnagogia. Hypnagogia is sometimes known as faces in the dark phenomenon because those who experience this state commonly report seeing faces while experiencing waking-sleep. Similar hypotheses have been put forward linking this condition to a number of other apparent paranormal experiences, including alien abductions, paranormal nocturnal visitations, and religious experiences such as contact with angels or demons.
My own view is that the experience could be any of the above. I am always struck how the aliens in popular culture, sometimes called "greys," frequently resemble depictions of demons in past ages; especially medieval paintings. I do not believe in alien abductions or aliens walking among us. I do, on the other hand, know that demons sometimes invade our environment and sometimes attempt to interfere in our lives and even to the point of possession.