I'll tell you what---The subject of possession takes a lot of heat, and it is one of those controversial issues, but I have personally had an entity partially meld with my field before, and I know it was one of the most traumatic things I have ever experienced.
These guys subject themselves to some SERIOUSLY dangerous situations.
It is really scary, and very uncomfortable. I went from feeling perfect to literally passing out and wanting to vomit in about a matter of 30 seconds. Not fun.
I was in the Oregon caves, taking a tour. I didn't even know they were haunted prior to going in. After the incident happened and the tour was over, I asked the guides if they had any ghosts in the cave system, and explained what had happened to me, and where in the cave. Two of the guides shuddered and had goosebumps. They said that a gold miner had slipped and blown his head off with his own rifle in the exact spot where I was overwhelmed.
I am a sensitive, although not highly developed yet. But yeah, I knew there was a presence, and I knew that it had temporarily attempted to attach itself to my energy field. I felt very sad, almost distraught, the second it happened, along with the nausea and extreme light-headedness. I was completely thrown off guard and not expecting it though, as I had no idea the place was even haunted. It knocked the heck out of me.