Another species of demon, one that's dear to my heart~ The Fear Demon. Vitya is actually rather special in that he is a full shapeshifter. Shapeshifting is an ability not connected to a certain species, so other types of demons can have the ability to shapeshift their entire body. Shapeshifters aren't illusions and generally do reconfigure their body shapes, but some other demons can create illusions of changing their forms. The illusion would be broken by successfully walking through it or falling through it, as you would feel it, but pass through.

As for Fear Demons, almost all contain illusion powers. They are some of the most dangerous demons, as they feed on fear, and the things people fear often include dying or harm coming to them or others, and while some Fear Demons strive not to hurt people and just trick them into their fear, other, like Vitya, will easily and happily tear someone from limb to limb, if not for the fear, then just for fun. As with most every type of demon, their horns are sensitive receptors, helping to pick up basic human emotion, and of course, they help to determine the way to get what energies they need from someone(like Vitya's first transformation being a huge spider because Valerie is scared of spiders. Don't tell him you know that).

Fear Demons, unlike most others, are typically genderless, with their genders being one thing they all have the ability to shapeshift at will, if only to allow for breeding.

I'm sure there's more info in my head on these guys too, especially Vitya, since he's a special case, but I am SO TIREDDD.