Forbidden Technique!
There were many reasons why a Ninjutsu, Genjutsu, or Taijutsu would gain the label of "Forbidden Technique."
Some were considered forbidden because they had a very high talent requirement or special circumstances that made it almost impossible for even the strongest ninjas to learn or master them.
Flying Thunder God Technique was one such example.
It required a high space-time talent, Grandmaster-level sealing jutsu proficiency, lightning-fast reflexes beyond human limits, and several other limitations.
Each one of those challenges was a hurdle that most ninjas could never overcome.
The requirements were so overwhelmingly despair-inducing that even if Senju Tobirama openly shared this ninjutsu with every single ninja in this world, it would be tough for even one person to learn and master it in each generation.
The fact that only Senju Tobirama and Namikaze Minato ever managed to learn and use the technique over a period of more than fifty years in the original Narutoverse spoke volumes.
Some were considered forbidden jutsu because those techniques posed a significant risk to the users' lives, on top of being ridiculously difficult to master.
For example: Spirit Transformation Technique
This technique literally allowed the users' souls to leave their bodies while still being anchored to their bodies and to invade other people's bodies to shock their souls and kill them.
Unfortunately, the moment the users' souls left their bodies, their bodies wouldn't be able to move, leaving them vulnerable to attacks.
In the original series, Dan Kato died for the same reason.
Some were considered forbidden jutsu or techniques because using those techniques, rituals, or sealing jutsu required unspeakable human sacrifices.
For example, the cursed sealing ritual that granted Hidan inexplicable biological immortality. Thanks to that secret sealing ritual that made people rave about Jashin's name, even if ninjas decapitated Hidan or stabbed his heart, he would still be alive, as if it were no one's business.
Though the success rate of this bizarre forbidden jutsu's ritual was very low.
Others were considered forbidden jutsu because they not only required living sacrifices, but they also straight-up violated moral and ethical boundaries, for example, desecrating the bodies and souls of the dead.
For example, the Impure World Reincarnation Jutsu.
This forbidden technique required the DNA samples of a deceased person to summon and temporarily reincarnate the soul of that deceased person by sacrificing the body and soul of a living person.
And then, there were ninja techniques that were labeled as forbidden because using such techniques usually meant sacrificing lifespan, which might even lead to a user's death.
The examples were: Dead Demon Consuming Seal, Eight Gate Release Formation: Gate of Death, and the forbidden jutsu Echo was performing right now.
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❇️Flashback❇️
Inside an old but meticulously organized hut, Echo, to be precise, the 19-year-old Echo, was sitting in a standard Japanese seiza position, resting his knees on a soft, flat pillow.
At that time, he didn't have the codename Echo yet. He had a proper name.
But it didn't matter. He had long abandoned his formal name and adopted the identity of Echo.
However, right now, he was nothing but a relatively talented young ninja with an average Jonin-level combat power.
Before him sat an old man in the same seiza position.
He seemed to be in his late 70s or 80s.
He had a decent chakra reserve, enough to put him in the upper tiers of the Elite Jonin-level Shinobi from a hidden ninja village of a major elemental nation.
But his real age was actually closer to 120, which was incredibly rare among the inhabitants of the Shinobi continent.
After all, it was still the Warring States Era.
The old man revealed a tattered scroll and slid it toward the young Echo.
Seeing confusion on the young ninja's face, the old man simply smiled and asked, "You are about to set off for a journey, aren't you?"
The young Echo was still confused, but he still replied, "Yes, sensei. You already mentioned a month ago that you have taught me everything you could. My strength has stagnated. To improve myself further, I need to see and experience a wider world."
"That's what you said. Didn't you?"
The old man, who was Echo's mentor, nodded slowly, "Yes, you are right. I have taught you everything I could. You have enough strength to protect yourself now, as long as you don't court death, that is."
The old man's wrinkles deepened as he let out a chuckle.
Then, he continued, "So, yes. It's about time you leave. I'm already old. I probably won't last more than a decade, anyway. The old injuries and the use of several forbidden techniques truly did a number on my old bones."
The old man's murky eyes rested on the tattered scroll for a moment before he added, "And this scroll here…it has my final inheritance."
"I hope you will never have to use it. But sometimes, you can't just help it. Nobody knows where the thread of fate will lead you."
"In such a moment, if this technique can save your or your loved ones' lives, it would be worth it."
Hearing this, young Echo grabbed the scroll curiously before asking, "What exactly is it?"
The old man sighed and replied, "It's a precious yet forbidden jutsu of immense power passed down to me by my teacher before his death. For us, Swift Release users, you can say it's a divine technique. It's the reason I'm still alive today, and also the reason I am in this extremely aged state."
"And now, my disciple, I am passing it to you. Learn it, master it, and use it wisely."
"The name of this jutsu is…"
❇️Flashback Ends❇️
Swift Release Forbidden Technique: Void Shattering Leap
As soon as Echo activated the technique, Swift Release chakra, more potent, more fundamental than anything he had used earlier, began to gush out from his chakra points (tenketsu) and directly affected the surrounding spatial fabric.
At the same time, he could also feel the side effects of the forbidden technique hitting him like a racing truck.
He felt his lifespan being siphoned rapidly.
At the same time, Echo couldn't help but think, the more lifespan I feed to this jutsu, the more powerful it becomes.
Usually, the Swift Release chakra can't influence the space. At most, it can manipulate the kinetic energy and oscillating force of the air particles. But with this forbidden jutsu, I can directly convert lifespan into a special chakra that breaks through the limits of my Kekkei Genkai.
I can make the space oscillate and ripple.
Anything that touches these violent spatial ripples will be ground to dust, erased from existence.
Of course, the price is heavy, too.
Just a single combat use of this technique can burn several decades of lifespan if the control slips.
But…hah!
My life, my vitality, is already slipping away.
Since I am dying anyway, I will pour all my remaining lifespan into this one final attack.
Nobody can survive this.
No one.
Hahahaha…I am sorry, Sensei. I am no good person. Nor did I find someone I cared about. I have no bonds left. In these nearly forty years of my life, I have done nothing that could make you proud.
Your legacy will end with me, too.
But that's okay.
I will take at least one life here with me, as my final tribute to you.
Echo's expression became increasingly vicious as those thoughts swirled in his mind. His eyes became more determined as he decisively sacrificed all his vitality, making the space around him quake and ripple even more.
Crack! Crack!
An odd sound of mirrors breaking, along with an uncomfortable screeching sound, could be heard as the space around him visibly distorted.
Such a phenomenon immediately alarmed Seimei and Reina.
Echo, on the other side, sensed that his forbidden technique had reached its peak stage and charged toward Seimei without any hesitation, all while destroying the space between them.
His speed was so fast that it looked like he had completely disappeared.
At least, that was what it would look like to most ninjas below Kage-level.
However, neither Seimei nor Reina was an average ninja.
They had some of the top-tier perception-enhancing Kekkei Genkai—Dojutsu Kekkei Genkai that allowed them to see through Echo's tricks without any surprise.
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Since Seimei could see the nature of his enemy's technique, he knew there was no dodging it without putting Reina or their supervising instructor's life at risk.
A technique that could use Swift Release Kekkei Genkai's kinetic energy manipulation trait to the extreme to generate spatial ripples, which could destroy both ninjutsu and physical objects with ease, wasn't something most people could dodge, withstand, or defend against.
Well, unless you had a way to really counter it.
Fortunately, Seimei truly had a way to counter it, and he had to thank his old man for it.
So, he activated the technique decisively.
Flying Thunder God Technique—Second Step: Guiding Thunder
Under Seimei's meticulous control, a large, pitch-black Flying Thunder God Seal with a faint blue pulsating glow bloomed into existence in front of him right before Echo's devastating punch, along with violent spatial ripples that could reach him.
In normal cases, a fuinjutsu seal hovering in the air would be crushed into oblivion the moment the spatial ripples touched it.
But the Flying Thunder God Seal was not a common fuinjutsu seal.
As soon as it came into existence and Seimei poured a large amount of chakra into it, spatial fluctuations, no less intense than those generated by Echo's forbidden technique, appeared around the Flying Thunder God Seal.
It was just that the main purpose of the seal's spatial fluctuations was to fold the spatial fabric and teleport anything that touched it to a coordinate fixed by Seimei.
And what would happen when two opposing spatial fluctuations meet each other?
Destruction, of course.
Crack! Crack!
BOOOOOOOOOOOOOOOOOOM…
Echo's fist, covered in destructive spatial ripples, met the intense spatial fluctuations of Seimei's Flying Thunder God Seal, leading to a loud and devastating spatial quake that wreaked havoc all around them.
Spatial powers clashed.
Echo's spatial ripples tried to tear the seal. On the other hand, the Flying Thunder God Seal's spatial fluctuations tried to control the space around it, take it under its control, and make everything, including Echo, who had entered its effective range, teleport to a designated coordinate.
The clashing spatial powers seemed to tear the space apart as several black cracks appeared the moment they clashed. The very fabric of space seemed to crack like a mirror.
The moment those spatial cracks touched the ground or a nearby rock, they were obliterated and sucked into those spatial cracks.
Fortunately, Reina already distanced herself.
And Seimei and Echo were protected by their respective jutsu, allowing them to stay unharmed even though the area within a radius of nearly thirty meters around them had turned into an absolute death zone.
Everything happened so fast that Echo was slightly in a daze.
When he finally came to his senses, he looked at Seimei with a horrified expression and asked, "What? How? How can you still be alright?"
Seimei smiled coldly, but he didn't say anything.
However, in his mind, he vehemently mocked the man before him.
Hehehe…did you, bastard, think that you could compete with me in space-time ninjutsu?
I can sense that you have little to no spatial talent. Even the spatial powers you are displaying are nothing but a byproduct of your forbidden technique.
In the end, unlike me, you have no control over space.
To create a slight ripple in the space-time fabric, you had to sacrifice your lifespan. And yet, you have no control over it.
So, what will happen when you meet someone with actual spatial talent who also has a proper way to utilize and manipulate space?
Of course, you lose!
As soon as these thoughts crossed his mind, Seimei increased his chakra output even more, allowing the Flying Thunder God Seal to release an even more intense spatial power.
Seimei's spatial fluctuations quickly suppressed Echo's spatial ripples.
And in less than two seconds, under Echo's bewildered gaze, he completely obliterated his spatial ripples.
At the same time, Echo also felt an uncontrollable pull from the Flying Thunder God Seal. He felt as if the very space around him had been constricted.
He couldn't move at all and could only be helplessly pulled into Seimei's seal.
Seeing this, Seimei smiled and said, "Well, goodbye!"
That was all Echo had heard before the seal completely sucked him.
And Echo's figure disappeared from the place along with the Flying Thunder God Seal.
A few milliseconds later, Echo reappeared fifty meters away from his previous location. His figure seemed to materialize out of thin air.
He couldn't help but look around blankly.
His brain seemed to short-circuit.
He muttered, "How did I get here?"
Echo looked at his hands in a daze.
The spatial ripples that enveloped his figure? Gone.
His chakra? Gone.
His lifespan? Gone.
He felt weak all over, and his vision slowly darkened.
He looked up and watched Seimei, the very kid he wanted to capture, and the one who, along with the other Uchiha girl, pushed him to this irreversible predicament.
But at this moment, he had no energy or desire to get angry.
There was no point.
In the end, he was a powerhouse who had nearly stepped into the realm of a Kage.
His mentality was solid, even if he was greedy and somewhat deranged.
Echo's vacant gaze finally met Seimei's glowing blue eyes for one last time.
And before long, his vision completely darkened, and he drew his last breath.
Seimei watched Echo die silently.
Thud!
He saw Echo's lifeless body falling to the ground with a thud.
His eyes and chakra senses told him the truth—Echo had died.
Seimei breathed in relief and looked down at his tattered clothes and several lacerations decorating his body.
Most of them actually completely healed, thanks to his immensely powerful physique and improved mastery over Yang Release chakra.
Soon, in five or ten years, he should also achieve a body full of vitality that would heal from even the gravest injuries almost immediately without his conscious effort, just like the Senju Hashirama in Shikkotsu Forest Sage Mode.
As Seimei was thinking of that, Reina appeared beside him and gave him a deep look.
Then she shifted her gaze to the dead Echo and tilted her head toward Seimei.
She asked, "Finally dead, huh?"
Seimei's lips twitched, but he answered calmly, "Yup. Completely dead."
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