Thunder in the Abyss of Fear (Shattering the Darkness with Thunder)

At the foot of the mountain, the air was filled with the bloody, chilling scent drifting from deep within the forest. Zenitsu Agatsuma curled up behind a massive old tree, pressing both hands tightly over his ears, but the sounds—those terrifying sounds that clung to him like maggots on bone and slipped through every crack—still stubbornly wormed their way into his mind.

The wind blowing through the forest was no longer a soothing rustle, but the eerie whisper of spider silk rubbing together, as though countless invisible threads were tightening. Faint, brief screams and muffled groans came from afar, impossible to tell whether they came from humans or demons, cutting at his nerves like a dull knife. Deeper in, the dense skittering of crawling arthropods seemed to rise from beneath the earth, making his scalp tingle as though the entire mountain were writhing. He could even "hear" the faint hiss of trees being slowly corroded by viscous liquid, and smell the growing stench of death on the wind, a mixture of rust and sickly sweet decay.

"I'm going to die... I'll definitely die if I go up there..." Zenitsu's teeth chattered softly, his face was as pale as paper, and tears streamed uncontrollably down his face, mingling with the snot smeared all over it. He stared fixedly at the ground, afraid to look up at the mountain crouching in the darkness like a giant beast. Fear wound around his heart like icy vines, squeezing so tightly that he could barely breathe.

He wanted to run, to get as far away from here as possible. The urge was so strong that it nearly controlled his every action. Yet his feet seemed nailed to the ground.

Because aside from the sounds of terror, there were other sounds—or, more precisely, images and feelings from his memories—clashing fiercely in his heart.

Tanjiro's face appeared before his eyes, always gentle and concerned. "Zenitsu, calm down!" "Take care of yourself!" There had been no reproach in his voice, only acceptance and trust. And there was Nezuko, the girl who stayed quietly in her box and occasionally gave him an innocent, curious look.

Moyu's face, usually expressionless, yet always appearing in front of them at crucial moments, resolving problems with strength great enough to put them at ease. His calm words before leaving, "Take care of yourself and catch up as soon as you can," had not been an order, but more like the trust of someone leaving his back in their hands. And there were those missing members of the Demon Slayer Corps, whom he had never met... They might still be alive, waiting for rescue in some dark corner.

"Wh-what am I doing here...?" Zenitsu's voice trembled with self-loathing. "Tanjiro... Moyu... that boar-head Inosuke... They're all up there fighting... and I... I'm hiding here like a useless piece of trash..."

Shame seared his soul like a red-hot branding iron. Fear and his sense of responsibility pulled at him wildly, nearly tearing him apart. He hated his own weakness, hated the ears that heard too many terrifying sounds, and hated the body that wanted to flee at the first sign of danger.

Just then—

"Aahhh!!!"

An utterly piercing scream, filled with despair and unwillingness, suddenly erupted from deep within the forest, piercing through every obstacle and stabbing with perfect clarity into Zenitsu's ears! It was not the roar of a demon. That voice carried a human's final emotions and pain, and it instantly shattered Zenitsu's psychological defenses.

Who was it? Was it Tanjiro? Or Inosuke? Or one of the other Corps members?

The scream was like a key, suddenly opening a valve in his heart that fear had long held shut. Image after image flashed through his mind: Tanjiro's resolute back as he shouldered the wooden box carrying his sister, Moyu's silent resolve as he charged alone into the darkness, and even Inosuke's reckless but never-yielding roars...

"No... no, no, no!" Zenitsu shook his head violently, sending tears flying. "I can't... I can't just watch... Even if... even if I really die..."

He crawled out from behind the tree on his hands and knees, trembling all over like a leaf in the wind. He looked toward the dark entrance to the forest, where the mountain seemed like a great maw waiting to devour people. Every rustle of wind or grass made his heart leap. The terrifying sounds in his ears grew clearer, every one of them screaming, "Danger! You'll die!"

But he still moved his feet, little by little. One step, two... His steps were unsteady and staggering, as though he might fall at any moment. He drew his Nichirin Blade. It trembled faintly in the dim light, reflecting his bloodless face.

"Tanjiro... Moyu... Nezuko... wait for me... I... I'm coming..." His tearful voice was almost inaudible, sounding both like a curse upon himself and a final attempt to bolster his courage as he plunged headlong into the thick darkness.

The moment he entered the forest, the terror multiplied exponentially. Visibility dropped sharply, twisted branches resembled demonic claws, sticky webs covered the ground, and the stench of rot and blood in the air was almost tangible. Zenitsu's extraordinary hearing became his greatest torment. The flood of sounds from every direction nearly burst his brain: the groans of the dying, the whispers of demons, the faint sound of spider silk tightening, and perhaps even the frantic pounding of his own heart.

He saw the first rescue point Moyu had left behind. The unconscious Corps member beneath the tree and the wisteria mark reassured him for a moment, but then he saw a corpse not far away, wrapped in webs and long since lifeless, and his stomach churned.

"Urgh..." He retched, tears streaming even harder, but he did not stop. He forced himself not to look at the grisly sights, staring fixedly ahead as he struggled to find his way by the faint traces and the weak scents of Tanjiro and Inosuke lingering in the air, stumbling onward over uneven ground.

The farther in he went, the more horrific the sights became. Corpses drained of blood, broken weapons, tattered Corps uniforms hanging from trees... Every discovery deepened Zenitsu's fear and weakened his legs. Several times, he wanted to turn and run, to escape this hell. But whenever the thought arose, imagining Tanjiro and the others in danger, along with the shame and self-reproach more terrible than death that he might bear if he fled alone, forcibly pulled him back.

"I can't run... I can't run... Grandpa... I'm sorry... I might... really die..." He muttered through his tears like a child trying to muster courage on a dark road, except this "dark road" was filled with real threats of death.

As he passed through a particularly dense thicket where webs hung like curtains, an intensely pungent stench, a mixture of decay and some acrid chemical odor, hit him in the face and nearly suffocated him.

"Well, well, look who it is. A little mouse that's lost its way and wet its pants in fright?" A slick, mocking voice came from the shadows ahead.

Zenitsu looked up in terror and saw an extremely bizarre "creature" slide down from a huge web coated in slime. The thing had the head of a young man, a twisted, cruel smile on his face, but his body... was an enormous spider covered in black hair! Eight jointed legs supported his massive frame, and his swollen abdomen gave off a nauseating stench. He was Spider Brother, one of Rui's "family."

"A-a demon!" Zenitsu screamed and instinctively backed away, only to trip over a vine and land on his backside.

"Hee-hee-hee, a coward like you can become a Demon Slayer? Has the Demon Slayer Corps run out of people?" Spider Brother slowly closed in, his compound eyes studying the trembling Zenitsu with interest. "Still, that's fine. With skin so soft and tender, you should make a nice 'collectible.'"

"D-don't come any closer!" Zenitsu scrambled to his feet and raised his Nichirin Blade, but its tip shook like a reed in the wind.

"Are you scared? Terrified? Hee-hee, you won't be afraid for long." Spider Brother opened his mouth, gurgled in his throat, and suddenly spat a glob of green phlegm trailing acrid white smoke!

"Thunder Breathing, First Form: Thunderclap and Flash!"

Extreme fear instead awakened the fighting instincts in Zenitsu's subconscious. With his eyes almost closed, he used the one technique he had mastered, relying on muscle memory forged by countless training sessions with his grandfather. Golden lightning flashed and vanished, and Zenitsu shifted several feet to the side in an instant. The glob of highly acidic, mottled poison phlegm grazed the edge of his clothes and struck a tree trunk behind him, immediately corroding a deep, sizzling hole into it.

"Oh? Interesting. You're quite fast." Spider Brother was slightly surprised, but his smile soon grew wider. "Still, being fast on your feet won't do you any good."

His abdomen squirmed, emitting a strange buzzing sound. In an instant, the "cocoons" wrapped in spider silk on the surrounding ground, on tree trunks, and hanging nearby burst open, and twisted "things" crawled out of them. They had once been human, but had now become half-human, half-spider monsters. Their faces retained some of their features from life, but their expressions were blank and their eyes vacant. Their bodies had fused with spider limbs, and their movements were stiff yet swift.

"Human-Faced Spiders," puppets transformed and controlled by Spider Brother's poison.

"Go on, my children. 'Mark' our young friend here," Spider Brother ordered mockingly.

"Waaah!" As dozens of Human-Faced Spiders surrounded him from every direction, Zenitsu was scared out of his wits and could barely hold on to his sword. He tried to use Thunderclap and Flash again to break through, but fear severely disrupted his concentration. His movements faltered, and his speed dropped sharply.

One Human-Faced Spider lunged, slashing his arm with a sharp leg. Another attacked from the side, a tiny stinger springing from its mouthparts and piercing his thigh.

"Ah!" Zenitsu cried out in pain. The puncture hurt terribly at first, followed by a sensation of numbness and burning that quickly spread to the surrounding area. At the same time, a cold energy bearing a chaotic consciousness entered his body through the wound.

"Hee-hee, the injection's complete." Spider Brother watched at his leisure. "Four hours. In just four hours, you'll become like them, one of my most obedient collectibles. Now, how do you feel? Aren't your arms and legs going weak? Isn't it getting hard to breathe?"

Zenitsu could indeed feel it. The poison took effect immediately. His strength was draining away, his vision was growing blurry, his breathing became rapid, and the sounds in his ears grew even louder and more chaotic. His swings of the Nichirin Blade grew weaker and weaker, and the lightning of Thunder Breathing dimmed.

"No... please... I don't want to become like that..." Zenitsu cried out in despair as the Human-Faced Spiders forced him back again and again. Several more wounds appeared on his body, and the poison spread faster.

Spider Brother admired his despairing expression as he slowly approached. "Give up, little mouse. There's nothing wrong with becoming a spider. At least... you won't feel fear anymore, hee-hee-hee."

More attacks came. Zenitsu barely managed to block them, but his Nichirin Blade nearly flew from his hand. Fear, poison, physical exhaustion, and hatred for his own helplessness... All manner of negative emotions engulfed him like a tide. His vision gradually darkened until all he could hear was Spider Brother's grating laughter and the skittering of the Human-Faced Spiders.

"Grandpa... I'm sorry... Tanjiro... Moyu... I... I still..." At the moment the last trace of his consciousness was about to fade, his extreme fear seemed to cross a certain threshold.

Darkness enveloped everything.

Spider Brother looked at Zenitsu, who had collapsed limply to the ground and seemed to have lost consciousness. He strode forward triumphantly, intending to bundle him up in silk and carry him away. But just as his jointed leg was about to touch Zenitsu—

"Boom!!!"

It was not real thunder, but the explosive roar of a certain force!

Zenitsu, who had been collapsed on the ground, suddenly sprang bolt upright without warning, in a way that defied the laws of physics! His eyes were still tightly closed, and the tear tracks on his face had yet to dry, but all his fear, weakness, and confusion had vanished. In their place was an absolute calm, one that could even be called... vacant. He was like an empty shell driven by a program.

Golden lightning, fiercer and purer than ever before, burst around him without warning! It was not loose arcs of electricity, but brilliant golden light condensed into substance, flowing like liquid! The Nichirin Blade in his hand hummed, its blade completely enveloped in dazzling lightning.

"?!" Spider Brother started in alarm and instantly retreated, gripped by an intense sense of danger.

But it was already too late.

"Thunder Breathing, First Form..."

Zenitsu's lips moved slightly. His voice was perfectly calm, without the slightest ripple, yet carried the resolve to cut through everything.

"Thunderclap and Flash: Sixfold Strike."

There was no preparatory movement, no gathering of strength. Zenitsu vanished from where he stood.

No, he had not vanished. He had become six bolts of lightning that erupted almost simultaneously, weaving into a devastating net of light!

Fast! Indescribably fast! It was a speed beyond the limits of what Spider Brother's compound eyes could follow, beyond even the speed of thought!

Several flashes of lightning passed, and the surrounding Human-Faced Spiders were cut down almost simultaneously.

Spider Brother only had time to cross his two toughest front legs in front of him and spew a large mouthful of mottled poison phlegm in an attempt to block the attack.

"Sssh!"

The lightning sliced easily through the highly acidic poison phlegm, vaporizing most of the liquid, and struck Spider Brother's front legs without losing momentum! The hard carapace, capable of withstanding slashes from ordinary swords, was cut apart before the power of lightning condensed to its utmost like butter before a hot knife! Intense pain followed!

"Impossible!" Spider Brother shrieked, retreating.

A concentrated, swift flash of lightning appeared behind his neck, as though it had crossed the distance of space itself, before his horrified eyes.

"Pfft—"

The soft sound of a sharp blade cutting into flesh.

All of Spider Brother's movements, expressions, and thoughts froze. Incredulous terror still lingered on his human head, and the light in his compound eyes rapidly dimmed. His enormous spider body paused, and then his head slowly slid from his neck. The cut was smooth and even, and hardly any blood sprayed out, because the intense heat of the lightning had instantly seared the blood vessels shut.

"Boom!" The headless body crashed to the ground and quickly turned to drifting ash.

The golden lightning slowly receded. Zenitsu appeared behind the place where Spider Brother had stood, still holding the posture of his completed slash. The lightning on his Nichirin Blade gradually faded.

The next moment, his body swayed, the golden light in his eyes rapidly faded, and his vacant expression gave way to pain and weakness. Once the poison in his body was no longer suppressed by Thunder Breathing, it instantly spread throughout his body with his blood, taking effect even more fiercely.

"Ugh..." Zenitsu groaned. Darkness came over his eyes, and he crumpled forward, losing consciousness completely and falling into a deep coma. His Nichirin Blade fell from his hand with a clatter.

In the forest clearing, all that remained were the lingering smell of scorching, the dispersing ash, and Zenitsu, unconscious and gravely poisoned. The earthshaking Sixfold Strike of Thunderclap and Flash seemed to have been nothing but a fleeting dream.

After an unknown amount of time, footsteps so light they were almost inaudible approached.

A petite figure appeared at the edge of the clearing. She wore a haori decorated with colorful butterfly-wing patterns, which seemed to flutter faintly and lifelike even in the dim light. She looked at the traces of battle, then at Zenitsu, who lay unconscious on the ground, his face beginning to turn bluish.

"Spider Demon poison... and traces of Thunder Breathing... He's badly hurt, but he's still alive. How tenacious." The girl's voice was clear and pleasant, with a trace of professional calm. She crouched down and quickly examined Zenitsu's injuries and poisoning, her delicate brows knitting slightly.

"He needs to be taken back for treatment as soon as possible." The girl deftly took out a roll of bandages and several pills, first giving Zenitsu a mild antidote before providing basic treatment for his most serious wounds. Then, her seemingly slender arms proving unusually strong, she easily hoisted Zenitsu onto her back.