The Greatest Assassin Gets Reincarnated in Tensura

The Eternal City stood proud, its architecture a masterpiece of both mysticism and shadow, blending abyssal grandeur with an eerie, otherworldly beauty. Towers of black obsidian and crimson glass spiraled toward the sky, streets paved with ethereal silver runes that pulsed faintly with energy. The entire city breathed a life of its own, whispering to its new inhabitants, welcoming its true rulers—the Abyssal Legion.

Having evolved into Demon Nobles, Archdemons, and Greater Demons, the newly transformed warriors roamed the streets in awe, reverence, and silent excitement. Every step they took hummed with newfound power, their very souls infused with the blueprint of the city Arion had transferred into them. Each demon understood the intricacies of its design, the hidden pathways, the secret chambers—

The Eternal City was not just a home. It was a fortress, a battlefield, a kingdom of shadows.

And now, they would be trained to master it.

As the demons explored their new domain, Arion gathered them once more. The atmosphere was electric with anticipation, their new forms radiating an aura of terrifying might.

Then, a gust of wind erupted from nowhere.

The air crackled, and the ground shook as an unseen presence manifested before them. Golden, slitted eyes gleamed in the dim light as a tall, imposing figure materialized beside Arion.

Veldora.

The Storm Dragon, in his human form, stood before them, arms crossed, exuding an aura of unimaginable power. His golden hair gleamed under the eerie glow of the Eternal City, his sharp, playful grin a stark contrast to the sheer dominance he exuded.

A ripple of unease passed through the ranks of demons. Even as Demon Nobles, the presence of one of the True Dragons was an overwhelming force to be reckoned with. Many instinctively lowered their gazes, their instincts warning them of the danger standing before them.

Diablo, however, grinned.

"Impressive," he murmured, his crimson eyes gleaming with intrigue. "To think that our Lord Arion has ties to such an entity… as expected."

Testarossa tilted her head, a rare smirk playing on her lips. "The Storm Dragon himself… this is quite the revelation."

Veldora laughed, placing a hand on Arion's shoulder. "Oh, come now, don't be so tense! Arion and I go way back."

Some demons exchanged glances, still processing what they had just witnessed.

Arion's voice cut through the silence.

"All of you will soon come to understand something," he said, his golden eyes sharp as they scanned the crowd. "Power is not simply about strength. It is about connections. About understanding the flow of the world and shaping it to your will."

He gestured towards the city. "This is your home now. Go. Find where you belong. You will have one night to explore. Tomorrow, your true journey begins."

With that, the demons dispersed, some still casting wary glances at Veldora, while others, like Diablo and Testarossa, remained intrigued.

The next day, the entire Abyssal Legion gathered in the heart of the Eternal City.

The plaza was vast, its ground made of black marble veined with pulsating silver energy. At the center stood a grand podium, where Arion, Veldora, Diablo, and Testarossa stood. Behind them, the banner of the Abyssal Legion billowed in the air, its insignia—a black serpent wrapped around a bleeding crimson moon—etched into the fabric.

The demons stood in perfect silence, their eyes locked onto Arion.

Then, he spoke.

"You stand here today, not as the demons you once were… but as something greater."

His voice was steady, commanding, yet laced with a dark promise.

"You have evolved, not just in form, but in purpose. And now, that purpose shall be refined."

His gaze swept across the ranks, piercing through each demon like a blade of truth.

"You are no longer mere warriors, no longer mindless forces of destruction." His tone sharpened. "You are shadows. You are whispers in the dark. You are the silent hand that shapes the course of history."

He lifted his hand, and the entire city dimmed for a moment, as if responding to his will.

"From this moment forward, your training will begin. You will not simply learn to fight. No—you will become something more."

He took a step forward. The air itself seemed to tremble.

"You will learn the art of assassination—true assassination. Not the crude, reckless killing of lesser beings. You will master the art of silence, of striking with such precision that no one will ever know you were even there."

Murmurs rippled through the ranks.

"You will learn deception," Arion continued. "To wear masks. To blend in. To become whoever you need to be to complete a mission. A beggar. A noble. A scholar. A simpleton. You will learn to play every role, every persona, every identity until you are nothing more than a ghost, a shadow, a legend whispered in fear."

His voice dropped to a lethal whisper.

"And when the time comes… when your target falls by your hand, it will not be with noise or fury."

The shadows around him deepened.

"It will be with silence. The most terrifying silence the world has ever known."

The demons stood frozen, captivated.

Veldora smirked. "Damn, that's chilling."

Diablo's grin widened. Testarossa's eyes gleamed with a newfound hunger.

Arion let the silence hang for a moment before continuing.

"You will not just be assassins. You will be butlers, spies, informants, tacticians. You will learn how to cook, how to serve, how to blend into the background while holding a dagger at the throat of a king."

His golden eyes burned with absolute authority.

"You will become the Abyss. And when the world speaks of us, they will do so in hushed whispers—if they even dare speak at all."

Then, he extended his hand toward them.

"Who among you is ready to walk into the shadows?"

A moment of silence.

Then—

A thunderous roar erupted from the Legion.

Their voices rang through the Eternal City, a declaration of their new fate.

They would become the most feared force in existence.

Arion smirked.

The Shadow Legion was born.

And with that, Arion divided the demons into two main legions—the Black Legion and the White Legion.

Diablo, the embodiment of precision, chaos, and cunning malice, took command of the Black Legion. His warriors would specialize in the darkest arts of assassination and psychological warfare, striking fear into the hearts of enemies before they even moved. They would become the silent executioners, the unseen death in the dark, eliminating key targets before wars could even begin.

Testarossa, the queen of efficiency and lethal elegance, took charge of the White Legion. Her warriors would be the perfect infiltrators, manipulators, and conquerors, striking from the shadows but also moving openly in high society, capable of toppling kingdoms from within without ever lifting a blade—unless necessary. The White Legion would breed rulers, masterminds, and puppet masters, ensuring that no kingdom could resist their influence.

Within each legion, the demons were further divided into elite squads, each mastering a different art of warfare, assassination, and subterfuge.

The Specters (Infiltration) – Masters of disguise, deception, and silent movement, these demons learned to blend into any environment, from royal courts to enemy strongholds. Their training involved:

Flawless impersonation – studying speech patterns, cultural behaviors, and mannerisms.

Stealth movement – navigating buildings without making a sound, evading detection even by magical means.

Social engineering – manipulating people into revealing secrets without them realizing it.

The Reapers (Elimination) – The purest assassins, experts in both close-quarters combat and long-range execution. Their training included:

Silent takedowns – mastering techniques from real-world assassin arts, such as garrote kills, nerve strikes, and instant blade kills.

Sniping and ranged assassination – perfecting bow, crossbow, and energy-based long-range eliminations.

Weapon concealment and quick-draw kills – drawing hidden weapons in less than a second for instant execution.

The Shadows (Conquest & Psychological Warfare) – Specialists in turning fear into a weapon. They trained in:

Misdirection tactics – making enemies believe they're surrounded when it's only one assassin.

Fear-based manipulation – using shadows, whispers, and illusions to drive their enemies to madness.

Terror tactics – leaving behind eerie signs of their presence to demoralize entire armies.

Political manipulation – learning diplomacy, governance, and controlling nobles from the shadows.

Battlefield strategy – mastering large-scale warfare, leading armies like kings and generals.

Economic control – ensuring that no kingdom could function without their influence.

Arion knew that mastering such skills would take years in the real world. To counter this, he used his power to make the city have a special effect like his special dimension—he called it "The Eternal Sleep".

In this realm, time moved thousands of times faster than the Cardinal World. A hundred years of training could pass within mere days in reality. It was a realm of endless night, shrouded in shadows, where only those permitted by Arion could enter.

Here, the assassins would train until they achieved perfection.

Arion activated his skill, Pandoria, and commanded it to scan his memories from his past life. Every technique, every discipline he had ever learned—from real-world espionage, martial arts, and assassination arts—was extracted and forged into a training regimen unlike any before.

Each assassin would undergo realistic, brutal training regimes modeled after the deadliest warriors in history.

Hand-to-Hand Combat

Krav Maga, Jujutsu, and Aikido-based techniques – rapid, efficient movements focused on disarming and instantly neutralizing opponents.

Blade combat mastery – training with daggers, throwing knives, and short swords for instant-kill precision.

Gun-fu and ranged weapon combat – learning to fight fluidly with both firearms and energy-based magic projectiles.

Stealth & Evasion

Parkour & silent movement – assassins had to move without a single sound, scaling walls, running across rooftops, and even disappearing into crowds instantly.

Light and shadow manipulation – learning to control how light reflects off them, making them almost invisible in plain sight.

Magical camouflage training – using illusionary magic to blend into any background.

Psychological Warfare & Interrogation

Interrogation resistance – enduring mental and physical stress to withstand all forms of torture.

Interrogation techniques – breaking enemies through psychological pressure, false hope, and mental exhaustion.

Theater of fear – planting misinformation, inducing paranoia in enemy forces.

Advanced Tactics & Conquest

High-level strategy training – studying wars, battles, and learning to control entire countries through subtle influence.

Economic warfare – controlling trade, supply chains, and funding secret wars.

Kingmaking – ensuring that their allies sit on every throne.

To infiltrate and conquer from within, the demons of the Black and White Legions were trained in the intricacies of human life, culture, and society. Their training was as intense as their combat drills—every skill sharpened to perfection, ensuring that no kingdom, no nation, no organization could ever detect them.

Each assassin underwent deep identity training—learning to discard their real selves and fully embrace an artificial persona.

They would live as different people for years inside the Eternal Sleep dimension—becoming farmers, nobles, scholars, and even beggars.

Perfecting accents, dialects, and mannerisms from various kingdoms.

Learning to cry, laugh, show anger, or affection on command—deception so deep that even magical lie detection would be useless.

Every assassin was required to master regional cooking styles, from peasant food to gourmet meals fit for a king.

The logic was simple: a true civilian knows how to cook their homeland's dishes. Any spy who fumbled a simple meal could be exposed.

They learned to prepare feasts, tavern meals, and even street food—enabling them to run inns, taverns, and noble kitchens if needed.

They studied market trends, currency values, and trade routes to operate as merchants in any kingdom.

They learned the art of haggling, negotiation, and black-market dealings.

They could run legitimate businesses, smuggle weapons, and control underground economies—all without suspicion.

They studied diplomacy, governance, and law—ensuring they could function as advisors or even rulers.

They practiced political negotiations, blackmail tactics, and courtly etiquette to control the elite from the inside.

They mastered the use of spies, bribery, and controlled scandals—ensuring they could destabilize or solidify power as needed.

They trained in manual labor, farming, smithing, and craftsmanship—learning to live as peasants, blacksmiths, or builders.

They mastered slang, folk songs, and traditions to blend into rural communities.

They could disappear into any social class—high or low—without suspicion.

To prove their mastery, every assassin had to live under a false identity for decades inside the Eternal Sleep realm.

Some lived as merchants, building businesses from nothing.

Others became nobles, climbing social ranks through strategy.

Some married, raised families, and lived normal lives—just to ensure they could maintain a cover without breaking character.

Only those who could fully integrate and thrive in human society graduated. Those who failed—who slipped up, broke character, or revealed their true nature—were retrained from the beginning.

When they finally return to the real world, mere days will have passed in the Cardinal World.

Now, no nation, no kingdom, no army could ever know who among them was truly an assassin. The Black and White Legions will transcend simple warriors—they will be merchants, nobles, politicians, criminals, and rulers.

They could kill a king, take his place, and no one would ever suspect.

They will be no longer just assassins.

They will be shadows that ruled the world.