Possession itself, is a disputed topic, some believe it can happen others not. But, here in this article I will write down what I have read over the years of those who believe they were possessed, including how it happened and the various types of possession.
To start, I will go over the process of possession. Now, most possession begins with an invitation of some sort that being, the use of an Ouija board, a summoning, or simply asking any nearby spirits to communicate with you. For example in a few cases I have seen, demons appearing as a fake spirit guide to lure in the person.
Upon, getting in the demon or negative spirit will begin to do things to consume the target. This can either be done by continuingly talking to the person about various things they are interested in, such that the person only desires to talk to the demon. In other words, it tells the person exactly what they want to hear, causing them to be consumed by the demon. (Consumption Method) The other way, is to cause strange things to occur such as objects moving, strange sounds etc. The main purpose of these is to induce fear into the person and weaken them overtime, as being in a state of fear releases a large amount of energy weakening the person and feeding the demon. (Weakening Method)
After this stage, depending if the person is consumed by it or weakened by it. The demon will try to enter the person. This is where various, in the consumption method, the demon will ask the person permission to enter them. After, this the demon will slowly take over the person and generally the person will not notice this until too late. Now, for the Weakening Method, a demon will continual weak the person, sometimes in very dramatic ways, until the person gives in or is too weak to resist the demon taking them over. Furthermore, these types of possession are some of the most noticeable ones out there, due to the things done to induce fear.
Finally, once the demon has infested the person's spirit, it will continue to feed off them until it either decides to fill out a purpose of some sort, usually something along the lines of trying to pull others under its control, summoning forth more demons, attacking other people, generally those considered threats and even trying to completely take over a person, which will often lead to their death.
Now, besides the standard forms of possession, a demon can also attach itself to an object or location. This although are generally the result of a cult or a possessed person placed another demon spirit within an object such that in the future, the demon placed within the object can find a new person to feed upon, and start the process it talked about above.
The last type of possession I would like to talk about is something I call the free willed possession that is the person knows it is a demon but allows it to be in control anyway. These types of possession are common in cults where members try to bring forth a spirit to communicate with it. As for one oddity with this type of possession is sometimes the demon will not try to take over a person's body, most likely due to the person doing everything it wants anyway. Basically, think about it as a split off stage of the consumption method, where the demon knowing it already has completely control will not take it any further.
For a quicker view here are the definitions of what I said above:
Consumption Method- In this method a demon will try to consume a person by telling them what they want to hear, such that a person only desires to hear what the demons as to say.
Weakening Method- In this method a demon induces fear in a person weakening them over time.
Human Possession- Varying on which method is used a demon will either take a person by asking to be let in (Consumption Method) or forcibly take over due to them being unable to resist (Weakening Method).
Environment or Object Possession- This is when a demon is placed within an object or place through cult or ritualistic activity. Here the demon will wait until it finds a target in which one of the above methods can be successfully used on.
Free-Will Possession- This is where a person does everything a demon tells them to, such that the demon as no need to enter their soul and instead remains in the surrounding area telling them what to do.