Der Ritter

Die Geschichte des Ritters or The Tale of the Great and Noble Knight, is a German folktale from around the Black Forest in Baden-Württemberg in South-West Germany.
It tells of a brave knight appointed by the church to slay a great evil who had been stealing children from a local village. It is thought to be one of the earliest examples of The Slenderman or Der Großmann as he would later be known as in Germany.
At the time he was refered to as Der Namenlose (The unnameable/unspeakable one), and Der Unheilige (The Unholy). 
The knight was given a special weapon, a holy weapon sent by God, said to have fallen from the stars themselves. Many believe it is forged of material originally from the place where The Slender Man came from, and that it is the only known weapon that can kill The Slender Man, jokingly named 'The Ender Blade' by fans of The Slender Man legends. Der Ritter's armour was also forged of this substance and gave him protection against a lot of The Slender Man's apparent psychic abilities.
The great knight took to battle and was able to severely wound the beast, but not kill it. It is said that the wound showed neither flesh nor blood, but something else. A darkness, but beyond any darkness of our world. A hungry, all consuming void was visible.
The knight knew this to be The Slender Man's true form and that he had only cracked the surface, causing it to become vulnerable.
Before the knight could take advantage of its weakened state, The Slender Man was able disappear in a flash of light and a loud crackling rushing sound like a bolt of lightning, and all that remained was a small wounded child. The knight believed that in wounding the creature, he was able free one of its many victims.
The boy named Benjamin was orphaned by The Slender Man, so the knight raised him as his own son.
It would be over a century before any further reports of encounters with The Slender Man were reported. It is assumed that Der Ritter weakened it so much, that it was unable to manifest itself in our world until some time had passed.