The Slenderman is said to be a mythical being which is said to be freakishly tall, wearing a black suit and tie with abnormally long arms and legs. He has the ability to stretch his arms and legs to inhuman lengths which gives him the name, "Slenderman." Other names for him are, Him, The Operator, Bundle, The Tall Man, The Thin Man, Der Schlanker Mann, Fear Dubh, Schlankwald, Tree Man, Slendy, Slender, The Pale One, The White King and Master. He behaves in an extremely passive aggressive manner, such as stalking his targets for months and years causing mental tortue to his victim for a reason we don't know. He uses his arms to scare and catch his victim, sometimes he makes tentacles grow from his back which he uses to collect his victim and to walk, but you never see him walk. Some people think he has the ability to bend space and time, to be able to teleport. He has ghost white skin with nothing on his face except for 2 empty eye sockets. He looks a bit like a mannequin (Some people prefer him with nothing on his face) If you are being stalked by Slenderman a sign of it is your nose bleeding with fever like the common cold. Your first encounter with Slenderman can cause effects such as nose bleeds, exhaustion, nausea, strong cough, vomiting, small signs of radiation poisoning, aches and pains and slight trauma of the eyes. If you have encountered Slenderman more than 5 times you might start coughing up blood, vomiting with blood in it, a lot of signs of radiation exposure, painful breathing, difficulty in swallowing and memory loss.

Here is a link to Slenderman Wiki.