In the back end of nowhere is a small door. The word "Rejected” is scrawled on this door in every language imaginable. There is no handle, merely a metal plate to push on to open it. A tall painfully thin man in black walks towards the door, his footsteps pounding on the flat grey ground half a second after each of his feet touches the ground. The door opens and he steps through it. Into a brightly lit room.
The door cannot be seen from the other side. Instead there is simply an airport terminal, with no visible way out. Creatures are splayed out on chairs, some sleeping on the floor, a few float silently through the air. As the tall man surveys the crowd they size him but no one makes a move to confront him. No one, that is except a hunched dark skinned humanoid in the corner. He shambles forward and introduces himself as a spinal tap vampire.
Speaking without words the tall man asks where he is and who all these people are. The vampire response simply “We are all rejected ideas. Mythologies that never had a chance to fully exist. So here we wait in perpetual limbo.” The tall one looks about with an eyeless face. Some of these creatures are variations on a theme, some are twisted monsters of fevered nighmares, and others are almost nothing but a thick fog in one area. Arrayed on the wall are photos. It points to them. “Ah yes,” says the spine sucker “the escapees. Somehow these bastards got free, reached the mortal realm and got just enough attention to exist. The rest of us? Just irregular creatures, rejected goods from the mind of man. Still at least a few made it. Mothman, the gray aliens, Jersey Devil and Springheel Jack. Hell some people say Cthulhu started here but we try not to think about that possibility.” The thin man questions his surroundings silently. His blank head betraying nothing but mild curiosity. “No easy way out my friend, but better than being killed I suppose. Still I would have loved a chance to transcend to common myth.” the vampire stares wistfully at his razor sharp claws each one a hollow needle. He shuffles back to his corner and the slender man examines the pictures. He is still only partially formed, a blank slate of a tall thin humanoid in a dark suit. The rest of him is a grey miasma.
Weeks pass and the slender man forms himself, in the hope, however slim, of getting out of this wretched timeless place. Nothing changes and most of the residents seem to far gone to care, they sit and rot slowly to nothingness, their bodies and minds breaking down to the component parts of mythology. But the slender man will not do that. With effort he claims the unformed nature of himself and grows to craft himself as an creature of fear, of loss, the loss of a child, of a primal terror of the modern age. Had he a face he would smile as he grows tendrils from his back and stretches to truly inhuman proportions. His body becomes a mockery of the world, a bald faceless businessman, black suit, white shirt, red tie.
A physical form crafted to stick in the mind the slender man tries to escape. It is slow going, finding a mortal mind to nest in. Six and twenty minds are contacted but each has no ability to spread his word or properly give credit to his horrendous nature and sinister purpose.
Months more pass, as time is counted in that grim place. The vampire now is nothing but a single needle on the ground, twitching weakly, but the slender man persists, casting his mind far and wide to find a nesting mind to spread from. Then success strikes in a flash. Someone, thinking themselves clever, crafts an image of the him, dark and terrible, and apparently real to the untrained eye. He whispers terrible secrets in their ear and they write them to go with the photo. Still this isn't enough but it's a toe hold. Within days though he is realized in this world. Fools mix with magi anonymously and spread tales they think they have made up about him. And so the universe slowly shifts itself. The Slender Man is real. He always has been after all. A child killing terror of the fog, invisible to all but the mad, the children, and the legions of Anon. His chest rises and falls as he fades from the terminal of rejected ideas and begins to raise hell in the land of mortals.