Some people have said ghosts can act like Angels. If that's true, then I suppose a ghost could act like a demon as well. There is a line someplace between ghosts, spirits and demons. I'm not sure where that line is, but this is a true story and was one of the spookiest things to ever happen to me.
When I was in the service I met a guy I'll call "Smith"*. I knew Smith for about a year before discovering (by accident) that he practiced some type of demonolatry... a kind of magic that involves invoking demons.
He was reluctant to discuss the subject with me, and to the best of my knowledge nobody where we worked knew about this except for me.
Eventually we were both assigned to attend some night classes together. I didn't have a vehicle, and we both lived in the same barracks, so Smith was letting me catch a ride back with him each night.
One evening, after class, I started a conversation about his witchcraft with him. It had been a few months since I first started asking questions, and he had opened up a little bit about it.
It was about 2:00am during the middle of the week. Our barracks were way out in the boonies, and the parking lot was empty except for us sitting inside his SUV.
He described some wild things he'd seen before he got involved with his black art, while around another group. And while practicing his magic, actually having seen demons he had invoked. Just as he'd finish with a story, I'd ask more questions. I was always pretty skeptical about this sort of stuff, but I found the stories compelling. I could tell he was getting tired and wanted to wrap up this conversation, but he was polite and kept answering my inquiries. After at least an hour of talking, the conversation began to head towards what he'd done lately.
He told me the only place he had, on the base, that he could practice his magic was inside his SUV. It was a big, old, jacked up, full size one... and that made sense, because we didn't have any privacy at the barracks. I don't remember the details, but in a nutshell he said he'd create some type of protective circle before invoking the demon for his spells. This was to protect him from the demon. And then the spell was supposed to force the demon to do his bidding, and then he'd banish the demon.
He said about a week ago he had messed up a ritual. That the protective circle wasn't correct, and he'd summoned a demon that he couldn't banish. I realized he had done the ritual right inside the SUV we were sitting in. He said people had seen him driving down the road, and asked him who the dark figure was sitting in the back seat of his SUV, when nobody was with him. I was starting to get a little spooked, but my skepticism was working overdrive. I was ready to call it a night, but he wasn't finished telling me the rest. He told me people at work had seen him make a "hissing" noise during the day for no reason, and he had no recollection of making the noise.
Just as he said that, and before I could reply, the radio antenna directly in front of the front passenger seat (right in front of me) started thrashing around, as if someone had grabbed it and was shaking it around hard. The sound it made was making was quite loud. I turned to look at Smith and his eyes were fixed wide on me, and then he opened his mouth and made a very loud hissing sound.
I nearly jumped out of my skin, but I kept my composure and just sat there as calmly as I could. Within a few seconds (but it seemed like eons) the antenna slowed down and stopped. I struggled for words, and said "Woah, that was weird.". Smith said "Yes it was".
I asked him why he'd hissed at me, and he said he didn't hiss.
I thanked him for talking to me, got out of the SUV and went in to the barracks.
*Smith was not his real name.