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Demonic possession  ... It is not always related to Karma - though it can be. It usually happens when a person is in a weak or vulnerable state. Negative spiritual entities are much like traditional predators we see in the animal kingdom. They go for the easiest targets and so when someone has gone through a difficult period in their life and is in a low energy state - they will see this as an opening.

These weak and vulnerable states …. can of course be brought on by poor karmic choices (drug use, sexual immorality, theft, incarceration etc etc) …. but that is not always the case. It could also occur when people are simply under a great deal of stress from work, have experienced great trauma or sadness in their life like the end of a relationship or death of a loved one or they are simply in a weakened physical state for some other reason.

When we are in a negative state of mind or under great stress - our energy body and state of being is more “in tune” with theirs. Thus they are able to more easily “hook into” our energy body to transmit their own negative energy into ours to then influence the person thinking patterns and emotional state. The basic premise is to continue bringing the person down to lower and lower states so they can take more and more control. Confuse their thinking - bring on negative thinking and behavioral patterns so they can then maintain this connection and control. - David Agry https://www.quora.com/Is-getting-possessed-by-a-demon-because-of-your-karma

When a person is possessed by a spirit and harms others, let’s say, do they create negative karma or is it just the spirit who creates karma?

It is not the case that the person is free of any kind of negative karma because they were under the influence of whatever this entity was. You can say spirit influence itself is the result of karma from the person’s previous lives, namely the karma of killing. And so because they have that habitual propensity for killing when they were vulnerable to this, the cycle continued. But they do not get off scot-free, karmically speaking.

A second type of spirit is called “forget-maker.” The first one is an “insane-maker” and the second one is a “forget-maker.” It’s a kind of dementia. How it manifests behaviorally is the person is demented to the point of being absolutely unresponsive, just sits, shows no indication of seeing, hearing, is utterly uncommunicative.

And there’s a third type and this is a mommo spirit who makes one “happy.” When this kind of spirit possesses an individual, the person becomes hysterical, just laughing for no reason, laughing uncontrollably for no reason whatever. Again, it’s a kind of mental derangement.

There’s a fourth type of spirit, which is called a roving spirit, and when you’re possessed by a roving spirit, you rove; you just can’t sit still, you’re always on the move, you’re always busy, you’re always just moving from hither and yon, and you can never just be at rest. InTibet, when you see a person who just can’t settle down, people would say jokingly, “Oh, this person must be possessed by a roving spirit.” - From an interview with Dr. Yeshi Dhonden, author of Health through Balance https://fpmt.org/mandala/archives/mandala-issues-for-2000/may/spirit-influence-is-the-result-of-karma-from-the-persons-previous-lives/