Discovered in 2009 The Slender Man (also spelled Slenderman) rampaged for 5 years, slowly declining in activity until 2016, the worst year for Slenderman attacks so far. Slenderman attacks took a steep dive after then and have been decreasing in frequency since.
Recounts of the Slender Mans attacks commonly feature him stalking, abducting or traumatizing people, particularly children.
He is depicted as a thin, unnaturally tall humanoid with a featureless head and face, wearing a black suit. He is described as being 8 feet tall or more. He is also know to cause "The Sickness" (also known as Slender Sickness). The Sickness, sometimes called Slender Sickness, is the name given to an apparent sickness consisting mostly of coughing fits and coughing up blood, along with occasional nausea, paranoia, vomitting, and extreme exhaustion due to an encounter and being stalked by the Slender Man. Many have also had bloody noses, fever, and sometimes amnesia. It is experienced mostly by those stalked by Slender Man.