Utilizing Cthulhu Mythos, the beginning prompt has a number of avenues to take off from. There's the Wagner Tower itself. A writer could choose to imagine a number of grim scenarios and histories of the tower.
There's a mysterious sound,
"dull blows coming from God knows what remote corner of the old building had decided to quit".
Blows of murder? Pangs of desperation, by a trapped victim?
From there it's a relationship between a doctor and patient. Psychoanalyst, specifically, so the weird relationship can be expressed in so many ways. Freud, Lacan, go nuts. Something's odd about Mendo, and he's done terrible things. He speaks of an old latin phrase about the meaningless void of existence. Then, with increasing tension, and a doctor on the verge of fear, he says it,
The King In Yellow
A fucking bombshell of potential to go from there. Wonderful set up.
The moonlight descended on the east side of the Wagner Tower like an ancestral bone dust. The ectoplasm of a vague awareness crossed a tenant’s mind seeking for oblivion: finally, the dull blows coming from God knows what remote corner of the old building had decided to quit and he would have slept. However, between the seventy-fifth and seventy-fourth floor, a particularly fine ear could have still seized an intermittent, stifled counterpoint of voices.
"I feel that this unusual condition is helping us bring out some interesting perspectives, Mendo." In breaking the silence, the psychotherapist's voice had soon lost its initial momentum.
"I want you to know that this time won’t be billed, go ahead if you feel like it." She tried to assume a playful expression. Hidden underneath her short suit jacket, Dr. Wallace's fingers were nervously playing with a fluorescent orange rubber bracelet.
"No-one is ever suspended, not even now with seventy-four floors of nothing underfoot..."
"Well, this is certainly a positive observation..."
"Shut up, you don’t know a shit." An almost calm remark, pronounced with a firmness that hit Dr. Wallace like a bucket of frozen water.
"Have you ever thought, doctor," Mendo continued, sharply spelling out his last word, "that the fear of emptiness, the horror vacui as they defined it in the Middle Ages, is nothing but the unconscious and desperate attempt to look away from the ultimate truth?"
Since the elevator had blocked its descent, the patient had confined himself to a corner on the opposite side of the entrance. His left leg was now dancing grotesquely, animated like it had a life of its own and in contrast with the cadaveric stiffness of his other body parts.
"I never thought of that." Dr. Wallace wisely responded in brief, observing for the umpteenth time the assistance number carved on the elevator control panel.
"Mmmm...” A growing moan on the other side of the narrow cabin.
The doctor instinctively thought of her daughter that night, when the wind had hit the fixtures of the old house in the mountains so intensely that it produced an endless banshee howl. The little girl had made a sound of compressed horror, just like that.
If only she had known, she would have never asked Mr. Anatoliy “Mendoza” Volkov, an extraordinarily subtle personality, to follow her downstairs after that emergency therapy session in her office. On the other hand, he was one of her first and most challenging patients. Furthermore, he used to pay awesomely.
"Because the void swarms." Now his eyes were on the doctor, sunken out and bugging out at the same time.
"Soon they'll free us, do you think you'll keep writing that song you were talking about?" Dr. Wallace ventured. She realized that the silk shirt was soaking with her acrid sweat.
"It's the Yellow King's dominion, he comes from the void, it's him who made me do those things. I did not want to." His whine ripped open in a sinister vocal of terror.
"Mendo .." She did not know what to add. Now the doctor's hand, behind her sweating back, was pressing the assistance button convulsively.
His wide open eyes. They had stopped staring at her and now they were pointing up, right behind her shoulders.
"Mendo, what's up?"
"The Yellow King. He's here."
Patricia Wallace, a practicing psychoanalyst for three years, did not believe in ghosts or spirits. Not even evil. Each human was a mere collection of experiences pasted over a fragile, but complex series of mental occurrences. This web of experience, mediated through biology, expresses itself in the linguistic patterns available. That is, the mind operates through the rules of language.
Mendo wasn’t evil. He was traumatized, and in his attempt to come to terms with that trauma, to overcome it, he’d done things. Horrible things. But it wasn’t her place to judge him for that. It was her place to understand it and help him, as his court ordered psychiatrist.
When Mendo was seven years old, he split his mother’s throat in two while she slept. After serving his time, he was found fit to return to society under a number of contractual conditions. Patricia Wallace was one of those. They’d met biweekly for three years. He had his peculiar habits, his peculiar phrases. But that was to be expected with a man battling schizophrenia and a childhood full of trauma.
Mendo was crazy, yes. But he’d improved. And he would continue to improve because that’s what medicine and science did. It helped people. It made sense of the world. And Patricia Wallace was a trained professional. Patricia Wallace had a grip on life.
So as she turned to look behind her, a sense of shame murmured beneath her sleek attire. Her suit jacket, grey and tailored at La Rico’s. $1,400. Her golden bracelet, hanging loose on her wrist. $8,000 gift from a failed marriage. Patricia turned around to counteract that primal fear, that crawling feeling demanding that you turn and look behind you because something is there and its coming for you. That childlike desperation to ensure your own safety.
Patricia turned and saw it. Hovering in the hallway on the seventy-fifth floor was a thin floating man dressed in tattered yellow robes. Patricia shrieked, but the sight had caught her with such surprise that it came out only as a whimper. She fell back against the elevator doors as they opened. Mendo, unfazed, stood watching. The figure approached her, a smirk evident under the shadow of the robes.
Patricia smashed the buttons on the elevator. Frantic, she hit the door button again and again and again. Click Click Click. The figure continued towards her. As the doors closed, it brought its face against them and she saw the skin was pale like the color of moonlight on a guillotine.
The doors closed and Patricia realized she couldn’t breathe. It’s just a panic attack, she told herself. Calm down. That wasn’t real.
She heard a voice. Mendo, on the other side.
“Have you ever thought, doctor,” Mendo asked. “the horror vacui…that void is a part of us all. Like the stars…over Carcosa…”
The floor gave way. A hole consumed Patricia.
I know, I know. I like the image of a void, a hole, a crater, symbolizing the doctor's fear of the void of life. I used it in a short story I wrote last week, too. I guess I'm just no the kind for subtleties...