Considering how much a person can change overtme, at which point do you become someone else? If some outside force where to take over your body, its possible that your sense of self would simply die off while your body countinues seemingly as if nothing have happened. Worse, you might be reduced to a prisoner in your own body, fully conscious but unable to make any decisions.
"someone else is in my head", Magenta thought, "and I do not know how much control I have left". He was guiding his combrades through this anchient cave-system like it was his childhood street, his decisionmaking was unconcsious, but he was never hesitant or second-guessing himself. Finally they reacched the inner sanctum, the fabled pillars of Ega-Xana.
"Are you sure that this will remove this... thing inside your body?" Johan asked.
"I have no idea", Magenta replied, "but it's worth a shot"
He positioned himself etween the pillars, just as depcited in the cave paintings and thouched each of the sacred seals. There was a vague humming and the faint smell of sulfur and incence, but apart from that, nothing was apprently happening at first. Then Magenta could notice part of the wall crawling, like the stratified layers of the rock had suddently turned into hungry maggots. This excited something within him and he could feel the same crawling sensation mirrored in his own flesh. his skin ruptured, but at this point, it was too late to scream. The semi-liqouid tendrils of an alien organism made its way across his body. Eventually these tentacles reached the rock, which rose to meet them. Then... short circuit. Magenta could feel his mind going unable to stop it. There was a moment of silence as the tendrils receeded and Magentas body hung lifeless in the air for a second before being dropped to the ground. His friends hurried to catch him.
"Are you okay?", Johan asked frantically, "please tell me you're okay!"
Magetnas body stirred, there was a quick blink before the answer, "I'm allright, I'm allright, you can let go off me now".

***

So yeah, I was listening to a podcast late at night about psychological horror (and the psychology of horror) and I was scetching this. I like horror, but I'm not easily scared. I know how to deal with something hostile, if its maller than me, I kick it, if its larger than me I run away. Its the uncanny and the unknown that's truly frightening, at least to me.