He looks like a man with extremely long, slender arms and legs. He also appears to have 4 to 8 long, black tentacles from his back. He wears a black suit strikingly , appears very thin and able to stretch his limbs and torso to inhuman lengths in order to induce fear and ensnare his prey.
He can hypnotized people, so they can't stop walking to him too.
He can be found in the woods. He trapped children and kill them. Besides he enjoys stalking people who are paranoid about his existence to frighten them. For that, we can imagine that Slenderman likes torturing his victims psychologically.
Brazilian Cave Paintings

Egyptian Hieroglyphs
The next known possible reference to the Slender Man comes from around 3100 BC in lower Egypt, with references to the "Thief of the Gods" or the "Thief of Kuk" becoming common place during the reign of Pharaoh Wazner. Hieroglyphic carvings representing the Thief were found in the pharaoh's tomb, who was rumored to have had some kind of encounter with the entity. The carvings resemble a strange figure with multiple upper limbs, one that has never been found in any other hieroglyph language.In Spain there isn't any story like that and if there are anyone I didn't ear about it.