Demonic Possession

Demonic possession is more common than you'd think. In most cases, demonic possession involves the demon ripping out the human's soul, and the demon fitting itself into that void. This means that the demon effectively seizes control of the possessed's free will--since the soul is where all one's morals and will lies, it effectively means that the demons makes all the choices affecting that human from then on out.

A demon can ride a human indefinitely, but the longer they remain in control, the longer that human's soul is separated from their body, and thus vulnerable. A demon who wants to reuse their host's body will thus often bind the soul to a soulstone to keep it safe and prevent degradation, or it being stolen by another demon, or it shifting into the Spirit World and becoming a ghost, or something along those lines. While the severing of the soul from the body is traumatic, that's nothing compared to the ravages an unanchored soul goes through, as it slowly loses its sense of self and its morality--both virtue and vice, the later of which is completely unacceptable for demons. A completely worn soul will result in the host coming back almost catatonic, with no moral responses or the ability to make choices of their own free will--rather useless for a demon, who needs sin. And it also basically results in death for the human, until they can slowly, over a period of years, reform their soul into something resembling that of other mortals.

Most weak demons can only possess a single body at a time, but there are demons who specialize in controlling multiple bodies, and demon lords who can command legions of the damned.

For a demon to possess an individual, they have to make an agreement with them. Well, not themselves, per say. Anyone with any degree of authority over that individual can also make the trade, such as a parent or a king or a landlord (how do you think you get so many cases of demonic possessions arising from people inhabiting a house with a bad reputation?) However, agreements made by others are less powerful, and easier to break, assuming the exorcist in question knows what they're doing.

Therefore most demons prefer a much more subtle corruption, essentially weaseling their way into the head of those whom they make contracts with, and becoming their "shoulder devil." This isn't a full possession, as the humans' soul remains in their body, but it does allow the demon to act as the human's conscience, and slowly tempt their chosen victims into sin. Now, this is entirely resistible, as the demon has no actual control over your body unless you let them have it--but the fact that so many people fall just shows that many would prefer to give into their darker instincts than resist.

Yet, it's actually possible for a stalwart soul to use this link in reverse, and redeem a demon, returning virtue into their souls. Well, it's theoretically possible. Nobody's ever recorded it being done before, actually. Mostly because demons probably aren't stupid enough to link up with a human who'd be able to try such a reversal. Also that's why demons tend not to link up with supernaturals... 'cause a fey or a jinn or a horror will probably know a thing or two about mind games themselves.

Given how common possessions are, it shouldn't surprise you that almost every culture out there has an exorcism ritual or two. I mean, it's one of the most common rituals out there--even non-mages such Catholic inquisitors know how to pull them off, to say nothing of the dozens of rituals the various monster hunters have adapted over the millennia.