My first encounter with a possessing spirit was when I was doing Focusing with a client some years ago. The client was in some distress and when we started Focusing there was something on the right shoulder. This was not a part of the client, which is almost always the case with Focusing, but it was an evil spirit that hissed at me. I did not know then how to do depossession and I reported the experience to my Supervisor on my Shamanic course. I journeyed on this possessing spirit and found that it was a soldier from an ancient war who had attached himself to various people over the years.
When we later learned about depossession on my course, we were told that all souls of dead people should go to the Upper World (I suppose some would call it Heaven). It did not matter what the people had done during their lives in the Middle World, they were all entitled to transcend to the Upper World. Unfortunately, many spirits do not make the transition. I have encountered spirits who believe that they have been too wicked to be entitled to transcend. There are also some who have died suddenly and do not realise that they are dead. In the modern world, with the decline of religions, people are ignorant of the existence of the Upper World and, therefore, do not know where to go when they die.
These souls drift around in the Middle World. This is where we get our ghosts and poltergeists. Often they will attach themselves to a living person to share in that person's energy and experiences. They are usually quite harmless but occasionally they can influence that person's tastes and decisions.
We have been taught to remove these spirits but also to make sure that they go to a place where they are not likely to attach themselves to another individual. If they are the souls of dead people we send them to the Upper World, where they can continue their existence.
Sometimes, when I am doing depossession, I come across alien beings. They are quite like "Q" who appears in Startrek. They have just come along to observe. I call upon the elders of their world to come and get them - and they do.
Of course, this is quite dangerous work and I have to take care that everything is in place to protect myself and those around me. I find the work fascinating.