unfortunately no story of demonic possession, only attacks. One in college a few years ago, one recently, although it could have been just me.
both times have made me step up my practice in protection magics and their various forms. They also always occurred late at night, and started with me being asleep.
The first one was during my freshman year, in the new freshman dorm. It was the end of the year, and I was sharing a room with this big computer gaming geek. He was going to be up all night playing in one of the computer labs, and I was exhausted and had gone to bed early. I started having this dream, where I was in the dorm, engaged in a game with water guns, and I have dived off into a bathroom to refill my gun. Although, suddenly it was like stereotypical old lady bathroom with lace and pastel hand towels and "Love is" figurines. So, I go to refil, and I start sweating. and it starts normal, then in seconds just runns up until its like standing under a showerhead. The bathroom starts to flood and I try to go to a drain or flush it away, and as I am starting to submerge suddenly I think, "WAIT, I AM DREAMING" and bam I am floating in this void, and there is this fleshy thing trying to swoop at me. At this point, I wake up and at the foot of the bed is this humanoid shadow, but with three dimensional features and somehow all the light in the room is outlining it and following it. My heart is thumping in my chest, and I couldn't breathe. It was advancing on me, and kinda looked like it was getting ready to taste me or something. Finally all I could do was just Pray ( I know kinda hokey but it was all I could think of) SO I start doing the Lords Prayer and it fled, after about three repitions. After that, I got out of bed, and cleansed the room and the next day warded it to high hell. I did not want a repeat of that.
The more recent one I was again sleeping, only this time I immediately started dreaming of being beaten up by this thing that was all arms and scales it seemed. After a bit I awoke again, terrified and feeling pummelled, and there was this small dark splotch hanging over me. I focused and performed a banishing pentagram and it vanished. My house used to be more protected, but its being remodeled and I think that it took out some of its defenses. So, now I just focus on protecting me, at least until this remodel is done.
So thats my demonic stories. Nothing fancy.

You name it, I had a problem and a run in with it.