Asklemmy

Surprisingly, a lot of the creepy media is fairly accurate, though extreme. Demons aren't prominent, we know they are angels who rebelled with Lucifer, and were cast out, so that would be their appearance, but in reference to possession, we basically have those that Jesus encountered and a few his apostles drove out in his name later on.

And what we see are people behaving almost like animals, screaming, shouting, with an inhuman strength to break chains or whatever locals have tried to contain them with, and inflicting a lot of self harm. There's a woman who would throw herself into fires, a man who had 100 demons in him (where "I am legion" comes from") who would throw himself onto rocks and off cliffs and cut himself, etc.

The more manufactured elements are the head twisting, anything to do with pentagrams, and honestly a lot of the hostility to others. People usually steered clear, but demon possessed individuals generally did more self harm than harming others, with cases where Jesus would meet them within cities, and they weren't surrounded by dead people or a panicking mob or anything. They also don't "haunt" or hunt people like they do in movies, but are usually extremely obvious.

Anyway, that's my experience purely from biblical account, off the top of my head, I'm sure others can add more detail or examples.