How do the demons possess the human body?

they feed on the individual's fear first, then when they are strong enough, they enter the body through that person's weakness

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Firstly, imagine a soul as a fire (a soul isn't really a fire, it's just a ball of energy) without this fire the body will die. A demon can temporarily (permanently if the person allows it on purpose) take the fire out. When this happens the body will do whatever it can to get a fire in them (this is your subconsience working so you don't have control). The demon then acts as a fire and the body normally agrees to taking in the demon. While the demon is inside it has control of most of the body. If the body rejects the demon at any time the demon will exit leaving the body without a fire, if the fire doesn't come back in time the body dies, so the body rarely ever rejects the demon by it's self. Normally the body's soul starts back up and the demon is rejected. The demon becomes weak after possessing the body so it needs to rest before being able to possess another human. Once a human has been possessed it builds up a resistance against demon possession.