The tall humanoid creature stood at the end of the alleyway, cornered by three others of his kind.
“Your killing must stop, Rendian.”
“It is my way, Aora, it is what I must do.”
“Then it is your way to die.”
The three raised their hands. Tendrils of flesh shot from them, wrapping around the fourth. They began to tighten, slowly crushing the one known as Rendian. But Rendian was strong, and tendrils of his own shot from him, wrapping around the limbs of one of his attackers. They tendrils tore the attacker apart, and moved on to the next one, Aora. But before Rendian could kill her, the other one withdrew his tendrils to save the entrapped one, tearing the tentacles away, leaving Aora to fall to the ground, dazed.
“Rendian, that is your final murder.” The savior, Borean, said.
“Borean, I will give you one last chance to leave. Take it, or I will kill you as well.”
The two faced off in the alleyway, tendrils slapping against the ground. Quick as a whip, Borean's tendrils shot towards Rendian, but Rendian was quicker, and his tendrils were already at Borean's throat, pulling him forward. He hung there for a second, face to face with Rendian, giving Rendian a second to speak.
“I gave you a chance.”
Borean's head was torn from his body and thrown out of the alley.
“NOOOOO!” Aora screamed, having gotten her breath back just in time to see Borean die. She jumped to her feet, tendrils shooting from all over her body to encompass Rendian. She squeezed until she could feel nothing left. She removed the tendrils, revealing nothing. Rendian had disappeared. Aora stood there for a brief second before running for help.
In the few brief seconds left to him, Rendian had searched for a way out. And found it in a weakness between his dimension and a neighboring one. He tore a hole in the two dimensions and escaped his death.
The caveman stood over his kill, catching his breath. All of a sudden, an image formed in his mind, of an extremely tall human, with long limbs, black and white clothes, and tentacles sprouting from his hands. Then the image was gone, and the creature was instead standing in front of him, exactly as it had been in his head.
Rendian looked at the thing in front of him, seeing that it was like a shorter version of him, with short stubby limbs and no tendrils, with only rags to cover him. Rendian nonchalantly reached out with his tentacles and choked the life out of the thing, dropping its dead body to the ground. He went off in search of more victims, and to explore this new world.
A MORE DIRECT VERSION
Slender Man hails from a different dimension (The so called S-dimension), where there is a planet full of the Slenderkind. He lives in the capital, called Slender-City for now, where he was wanted for killing at least three others of his kind, not including the two that he killed in his escape, making him a sort of serial killer (Slender-Killer?). He was cornered by three of his kind, Aora, Borean, and a third that I don't want to name, all of whom were tasked with finding him, and killing him if necessary. They cornered him, and he killed two of them, escaping when the third, Aora, overpowered him. He escaped through a weakness in the dimensional wall between his dimension and our own. He escaped, and promptly went back to killing the crap out everything. And that's where we are now, a couple thousand years, and bodies, later, with him still on his killing spree. This origin story explains many things about Slender Man.
- His speed. He likes to pop up in front of you when you turn away for a brief instant. This can be explained by him never really leaving the time field of his dimension. Time flows extremely quickly in the S-Dimension, making him appear to be faster than us. It's not so much that he's faster than we're super slow compared to him.
- Why we can't kill him. See above. It's entirely possible that if a human were to escape this dimension, we would become invincible just like Slender Man. Quite literally, the only thing that can kill a Slender Man, is another Slender Man.
- Why there's only one of him. There's only one of him in this dimension. The rest of his kind are back in his dimension.
- Why we have an inborn fear of him. He's been around for thousands of years, and we see him when our eyes are closed. He came from one mans mind, and is now in all of ours.
Of course, its only a matter of time before another Slender Person jumps to this dimension, either by accident, to escape capture (Like Slender Man did), or to come kill Slender Man. Which would make an awesome story...