Demon Slayer : The Cursed One.

[A Year Ago]

Inside a metallic house nestled among the other ragtag settlements, a young woman who appeared to be in her early twenties worked intently at a cluttered workbench. 

Vials of glowing liquids which bubbled in beakers and complex instruments surrounded her as she mixed chemicals with precise movements. 

A faint hum from an alchemical device filled the room being accompanied by the occasional hiss of steam. 

The woman's dark green hair, tied neatly in a ponytail, clung to her forehead with sweat. She paused, straightened her posture with a small groan, and wiped her brow with a cloth tucked in her belt. 

Setting the cloth on the table, she leaned her weight against the workbench, her fingers tapping rhythmically on its metallic surface.

Her gaze drifted to a frail boy sitting cross-legged on a neatly made bed across the room. 

The oversized but clean clothes hung loosely on his gaunt frame. Books were scattered across the bed, one open in his bony hands. He hunched slightly as he read, his hollow cheeks and sunken eyes giving him a fragile look.

Sensing her eyes on him, the boy raised his head slowly, his hollow gaze meeting hers. He didn't speak, nor did his expression change. His hands remained on the book as if waiting for her to begin.

She pushed off the workbench and strolled over, then perched on the arm of a nearby chair while crossing one leg over the other, and let her voice spread through the air. "Finished reading, Kai?"

Kai shifted his weight slightly and 

replied, his voice even, "I did." 

"All of them?" she pressed.

"All of them," he answered again.

Her smile widened as she twirled a strand of her ponytail around her finger. "Then tell me, when did the demonic invasion start, and what changes did it bring to humanity as a whole?"

Kai tilted his head slightly, pausing as if considering an appropriate response. Then, in a flat tone, he replied: "....You sound like the teachers that I've read about in books, Lisa."

"Blunt as ever I see," said Lisa as she chuckled softly, amused by the unexpected retort. But her amusement faded when the boy began to speak again.

"....Approximately a millennium ago. I cannot determine an exact timeline, as the records from the books I have read differ slightly in their details."

Lisa's smirk began to waver.

"As for the changes to humanity," Kai continued, "they are too numerous to count."

Lisa's emerald eyes narrowed slightly as she removed her medical gloves, never breaking her gaze from the boy.

"Doesn't matter. Tell me a brief summary for all of them," she demanded, crossing her arms.

Kai tilted his head slightly, then began again. "....Starting from their emergence. First, the beings known as ghouls emerged. They were the earliest manifestation of the spreading corruption. The ghouls were mindless creatures, driven by their insatiable hunger for flesh. When they did any damage to humans, the infection on the former would spread to the latter, turning their victim into another ghoul within hours."

He paused briefly. "Ghouls were resilient compared to ordinary humans but far from invincible. They could be killed if struck in vital parts. Their bodies would eventually rot if they didn't consume any flesh, weakening them over time. This natural decay made them less of a long-term threat."

"And yet, they still overwhelmed humanity. Care to explain the reason?" Lisa said, her voice devoid of sympathy.

"Demons," Kai replied. "While humanity struggled to adapt, the demons appeared soon after. Unlike ghouls, demons were far more intelligent, powerful, and predatory. They did not rot, nor were they mindless. Instead, they possessed abilities that defied natural laws for the humans of that era and unique abilities that allowed them to manipulate elements, create illusions, or reshape their surroundings."

"That ability is known as blood demon arts," Lisa said as she leaned forward slightly, her emerald eyes glinting. "Now, what made demons different from ghouls, aside from their power?"

"Demons were nearly immortal," Kai answered. "Unlike ghouls, they could not be killed by conventional means. They were immune to aging and disease, and their regenerative abilities allowed them to heal from almost any wound. However, their most critical difference was their weakness in sunlight. While ghouls could roam freely during the day, demons were forced to hide in darkness, as sunlight would incinerate them almost instantly."

"And that's when humanity learned to fight back," Lisa said, more a statement than a question.

"Yes," Kai continued. "Early in the invasion, humanity discovered the demons' vulnerability to sunlight. However, an organization from an ancient country known to be Japan uncovered a unique method to kill them, that is decapitation using weapons forged from special ores which can absorb and emit the properties of sun. These materials, when used to craft specialised alloy, could kill a demon." 

"They were called Nichiren blades to be exact, although they crafted other weapons based on personal preference," Lisa said, smirking slightly. 

"The ores were called Scarlet Crimson Iron Sand and Scarlet Crimson Ore, are discovered on high mountains, always in sunlight, made into weapons that can easily cut their necks, halt regeneration and turn them into ashes. But…" she paused, her gaze hardening, "even those weren't enough, were they?"

Kai shook his head. "The initial implementation was effective at first. Slayers began striking demons in the shadows….But the materials to craft Nichirin blades were scarce and the process was complex. So, mass producing them was simply not possible. Furthermore, the demon slayer corp also introduced Breathing Techniques to enhance human strength and speed, allowing them to match demons in battle. These techniques required compatibility, time, intense discipline and physical conditioning, meaning only a small percentage of humans could master even the basics. For most, it was beyond reach."

He continued, his voice detached. "Even with their early victories, humanity's lack of coordination and the overwhelming number of demons led to their gradual decline. Strongholds fell, one after another. The demons adapted, becoming more cunning and resilient. Eventually, humans were pushed to the brink. Their cities were razed, their armies slaughtered, and their leaders turned on each other in desperation."

Lisa exhaled softly, her expression neutral. "And then they turned to the big guns," she said knowingly.

"Yeah," Kai replied. "Even the most dreaded weapons of humanity, known as nuclear weapons, were deployed as a last-ditch effort against the demonic threat. The logic was sound, that is, the nuclear explosions generate extreme heat, a massive shockwave, and intense light that could mimic sunlight, potentially overwhelming a demon's regenerative abilities."

Lisa tilted her head, intrigued. "And yet, they failed. Why?"

Kai's dull eyes locked onto hers as he began to explain, "An atomic bomb can vaporize anything in its immediate vicinity, and the heat generated can reach millions of degrees Celsius. Many demons and ghouls were incinerated in the blasts. The intense heat and light overwhelmed their regenerative abilities, and the initial strikes did offer humanity a brief reprieve."

He paused, seemingly organizing his thoughts before continuing. "However, not all demons perished. Those that survived the explosions, either by sheer resilience or by being on the outskirts of the blast zones, became a far greater threat to humanity. Their exposure to the radiation and extreme conditions triggered mutations, making them far more dangerous than the normal demons, thus making the situation worse."

The woman giggled slightly, her emerald eyes narrowing with a sharp glint. "They fucked up, sooo bad."

Her voice held an edge as she continued, "The explosions also wiped out millions of humans along with demons, destroyed critical infrastructure, and rendered vast swathes of land uninhabitable. The demons, though? They merely adapted. The few who were destroyed by the bombs were far outnumbered by those who emerged stronger, more resilient to artificial light and heat. Nuclear fallout crippled human armies, but demons, immune to its effects, roamed freely through the irradiated wastelands, preying on survivors."

Her fingers tapped rhythmically against the edge of a nearby beaker as she spoke, her voice growing colder. "Humanity's fractured state only made things worse. Nations failed to unify in time to mount a coordinated defense. Nuclear strikes were sporadic, each country desperately trying to stem the tide on its own. The absence of coordination left strongholds open to attack."

She paused, her expression darkening. "Instead of buying time, the detonations only accelerated humanity's decline. They destroyed their own resources, their own defenses, and left themselves helpless in the face of an enemy that thrived in the chaos."

The woman's voice dropped to a murmur,"The only lesson humans ever seem to learn from history is that we never truly learn from it. We repeat the same tragedies, over and over, like clockwork."

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[A/N : If you guys want me to update faster, throw those shiny power stones on me. (I am not masochist btw.)]