Is it normal that a demon or "shadow person" follows me?

Alright, ever since I was in about the 3rd grade a demon or "shadow person" has been following me. It started after a friend of mine told me about it in detail, and everybody else who's been given details on it have started to see it as well. As a favor to everyone I'm not going to describe it. Anyway, this thing torments everyone who knows more than the fact that it exists. It doesn't just appear and vanish like most peoples stories say, it actually harms us physically. At one point it had progressively scratched my arms until they were blood red and bleeding in a few spots. Me and my friends who see it call it the "black thing." During one occasion it actually shook the axe in a friend of mines room. This happened after a period of time when it was appearing so frequently it was getting quite annoying, the black thing had actually knocked me into the counter and knocked out one of my teeth. Alright, i know this isn't normal, but does anyone know what we should do? And has anyone had a similar situation? By the way, I do NOT drink or use drugs of any kind. So please be nice and help. me out