First, they look like real shadows—flat and gray; moreover, they hide in them, too, even in shadows moving up to and including 75 miles per hour. When they move, they slide along walls and floors, similarly to shadows.
Secondly, they can mimic your shadow, matching almost perfectly your every move.
Almost.
Skill varies from shadow demon to shadow demons; and, of course, no demon is perfect, making the occasional misstep in their performance inevitable. It is at these times that the trained eye will spot a shadow demon posing as the real thing.
Like mirror demons, shadow demons don't always mimic a person exactly. Their most common imperfection is appearing after a person has been stationary for some time, and/or exiting before a person does, or when a person has only moved slightly. They also trail just slightly behind a person's movements, sometimes missing a matching pose altogether when in a situation where matching close enough to pass as a shadow isn't possible.
I can think of only one scenario where that could happen, Specifically when the shadow demon is aware that a trained eye has pointed his camera at it, and it unexpectedly stymied that it could not mimic its subject close enough to pass as a shadow for people who don't know what they're looking at (or believe in shadow demons).
I know this because it's the only explanation for what is shown in a video I made while demons of nearly every variety were parading through my home one night, as usual. In the video, you can see a shadow demon lagging behind the subject—a woman—and otherwise doing a pretty poor rendition of her shape. While that might have been obvious enough to me, others probably won't be convinced Until they see the shadow demon mimic the woman drinking from her bottle almost a full second late—and without the shadow of the bottle:
Clearly, the shadow demon didn't have a shadow-of-a-beer-bottle demon on hand to pick up at the same time the woman did; also, shadow demons are apparently unable to produce shadows of objects it does not readily possess. So, in this case, the demon simply lifts it empty hands as the woman lifts hers to take a drink.
Greater Ashland Paranoral Research comments on this video
On its Facebook page, Greater Ashland Paranormal Research wrote: "Hi James. We haven't had a chance to look at all of the vids or photos yet but have checked out some. I have to say the shadow-lady one was really interesting. It's hard to tell what's going on because it's difficult to see the perspective in light/shadow-play on the angle of the wall to the woman's right but at first-look it does look like the shadow isn't fully tracking well with her movements and you can see it move before her at one point, then she moves and it lags behind her, and it does appear the "shadow arms" go up with nothing in them. Unless there is someone else in the room, just off-camera, mocking the woman's movements, you could possibly have something there. We would like to see a higher-resolution of that video footage if it's available."
Greater Ashland Paranoral Research comments on this video
On its Facebook page, Greater Ashland Paranormal Research wrote: "Hi James. We haven't had a chance to look at all of the vids or photos yet but have checked out some. I have to say the shadow-lady one was really interesting. It's hard to tell what's going on because it's difficult to see the perspective in light/shadow-play on the angle of the wall to the woman's right but at first-look it does look like the shadow isn't fully tracking well with her movements and you can see it move before her at one point, then she moves and it lags behind her, and it does appear the "shadow arms" go up with nothing in them. Unless there is someone else in the room, just off-camera, mocking the woman's movements, you could possibly have something there. We would like to see a higher-resolution of that video footage if it's available."
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