Here's a theory on Slenderman...

There was a man who was a test subject for a group of scientists to do their "experiments" on. The man as a result took drugs that increased his hormones level, which then made him exceptionally tall. One day the scientists were testing their new hypnosis liquid that (in theory) would enter the brain and strengthens the cells that makes you obey however the effect was something different...the mans face started peeling off and soon all that was left was a head with no features. This angered him and so he attacked the scientists who had done it to him but there was no need. The hypnosis worked and when the surprised scientists look at him they were put into a hypnotic trance. Thats when he killed them.

That was the day Slenderman was born.

Later

The man retreated to his dark little home away from society. But due to this he became crazy. With no one to keep him company, not being able to see, smell and hear. With every passing day he became crazier and crazier, so warped that he started kidnaping little children and taking them to his lair where their parents would never see them again.

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