Deep within the desolate mountains of the Land of Bears, two figures were locked in combat.
The combatants were Diarmuid and Kakuzu.
However, rather than a life-or-death struggle, it looked more like an exchange of ninjutsu techniques.
"Fire Release: Fireball Jutsu!" Diarmuid's hands blurred as he wove signs, eventually opening his mouth to exhale a massive sphere of flames.
The Fireball Jutsu was originally a signature technique created by the Uchiha Clan during the Warring States Period. However, it wasn't a particularly complex Fire Release technique, nor was it a Secret Jutsu that required specific bloodline traits to execute.
Consequently, over time, the technique spread throughout the shinobi world.
Combined with its decent power and ease of use, it became a common, standard Fire Release ninjutsu. Practically any ninja with a Fire-natured chakra affinity could perform it to some degree.
This demonstrated the immense influence the Great Clans had on the shinobi world. Many ninjutsu were developed by them; civilian ninja simply lacked the vision and depth of knowledge to achieve such feats from scratch.
And these were merely the techniques that had leaked out. The ones kept hidden were the truly terrifying ones.
In Diarmuid's memory, Konogakure was a gathering place for such clans. However, starting from the Second Hokage, Senju Tobirama, there was a deliberate effort to centralize power and suppress the influence of the Great Clans.
This led to the existence of that formidable scroll: the Scroll of Seals.
The Scroll of Seals essentially compiled nearly all of Konoha's ninjutsu, including Forbidden Jutsu. Aside from the specific Secret Jutsus and Kekkei Genkai of individual families, it could largely be called the "Ninjutsu Encyclopedia."
This scroll was truly brought to completion during the reign of the Third Hokage, Sarutobi Hiruzen.
While Hiruzen did not possess a Kekkei Genkai, the Sarutobi Clan was itself a storied Great Clan of the shinobi world. The old man was a powerhouse who had studied under the Second Hokage and mastered all five elemental natures.
Known as the "Professor," simply put: if it was a conventional ninjutsu that didn't require a Kekkei Genkai as a medium, he knew it!
It was utterly ridiculous.
For a long time, after the fame of the First and Second Hokage had faded into history, Sarutobi Hiruzen was hailed as the "Strongest Hokage."
While that was certainly an exaggeration, from a pure ninja perspective, excluding those who "cheated" with god-like powers, Hiruzen truly stood at the pinnacle of what a shinobi could be.
As Diarmuid's Fireball Jutsu swept forward, Kakuzu remained unfazed. He didn't even bother weaving signs. The Earth Grudge Fear technique was unique; it allowed him to release chakra directly from his hearts to manifest ninjutsu.
The moment the fireball approached, Kakuzu merely performed a symbolic sign with his hands and spat out a massive deluge of water.
"Water Release: Wild Water Wave," Kakuzu barked softly.
Ninjutsu clashes are essentially confrontations of elemental attributes. Water Release holds a natural advantage over Fire Release. Forget a C-rank jutsu like Fireball; even B-rank techniques like Giant Flame Bombs would be neutralized by Wild Water Wave.
However, the moment the fireball was extinguished, the earth beneath Kakuzu's feet began to churn.
Before he could speak, the mountain soil surged upward, threatening to entomb him.
With a light tap of his toes, Kakuzu leaped into the air, flipping gracefully to land on a nearby tree trunk.
Before he could even throw a shuriken, a surge of dark energy erupted from his shadow, attempting to entwine him.
"Hey, that isn't ninjutsu! We agreed you wouldn't use that strange Kekkei Genkai of yours!" Kakuzu grumbled in annoyance. He kicked off the branch, disappearing in a flash of Soru-like speed with a Body Flicker Jutsu to evade the darkness.
It had to be said that in terms of reaction speed, ninjas were significantly more impressive than the average experts Diarmuid had encountered in the world of One Piece.
"Hahaha! Water Release: Water Severing Wave!" Diarmuid wove another set of signs and exhaled a high-pressure jet of water toward Kakuzu's landing spot.
"Hmph!" Kakuzu snorted. Seeking to reclaim his dignity, he chose to counter Diarmuid's Water Release specifically with Fire Style.
Forming the Tiger sign, Kakuzu roared, "Fire Release: Searing Migraine!"
In an instant, the entire field of vision was consumed by fire. Like a sea of flames, it swallowed everything in its path, instantly evaporating Diarmuid's Water Severing Wave.
As the inferno surged toward him, Diarmuid simply extended his arm and whispered:
"Black Hole."
The dense, dark power of the Dark-Dark Fruit erupted from his arm, blanketing the massive sea of flames in the blink of an eye. Within a few breaths, the B-rank jutsu was gone.
If not for the scorched earth beneath them, one might have thought the fire had never existed.
Kakuzu watched this with a dissatisfied expression. "Your Kekkei Genkai is simply illogical."
It had been over six months since Kakuzu accepted Diarmuid's employment and became his Sensei.
During this time, Kakuzu had stayed by Diarmuid's side, teaching him to master the power of ninjutsu. Simultaneously, he had grown familiar with the powers of the Gravity-Gravity Fruit and the Dark-Dark Fruit.
Following his own logic, Kakuzu categorized them as types of Kekkei Genkai. Diarmuid didn't bother to correct him.
If I told you I was a Devil Fruit user, you wouldn't understand what the hell I was talking about anyway!
So, he tacitly accepted the "Kekkei Genkai" label. Consequently, the combination of the Dark-Dark Fruit and the Gravity-Gravity Fruit was given a new identity: "Abyssal Release." [1]
It sounded a bit plain [2], but Kakuzu could attest that it was incredibly troublesome to deal with, a perfect balance of offense and defense.
In these six months, Diarmuid had mastered the nature transformations for Fire, Water, and Wind.
Whether it was due to natural talent or some other factor, he had found the initial entry difficult, but once he understood the basics, his progress was meteoric. He could use C-rank ninjutsu almost instantly, and B-rank techniques only took a few days of study.
A-rank techniques, such as the Great Waterfall Jutsu, still required dedicated practice and weren't yet second nature.
Initially, Diarmuid was surprised that he could master Water Release at all. As a Devil Fruit user, he was naturally incompatible with water.
Yet, he could actually manipulate it.
Furthermore, Diarmuid discovered he could now swim. The "Hammer" curse of the Devil Fruit seemed to have vanished.
This was entirely unexpected. When he first arrived in this world, he had tested it; everything had been the same as the One Piece world—he had zero resistance to water. Even though the water in this world lacked Pyrobloin cells, it still rendered him unable to swim.
Diarmuid had even mentally prepared himself to treat Water Release as his ultimate weakness.
Then, he had mysteriously "recovered."
After much contemplation, Diarmuid had a theory: World Assimilation. Gradually, this world was adapting to him, assimilating him, and stripping away the anomalies he brought from his original realm.
His powers could be integrated, but a curse with no local foundation was simply "re-written" out of existence.
Or perhaps, as Diarmuid suspected, this world was benevolent and inclusive, rather than seeking to reject him entirely.
Regardless, Diarmuid had successfully integrated into this world. The internal suppressions he once felt had dissipated significantly.
That said, his Haki still didn't flow smoothly. At this stage, he was completely unable to utilize Conqueror's Haki Infusion.
At the same time, he could feel both the Dark-Dark Fruit and the Gravity-Gravity Fruit undergoing changes. This was uncontrollable; it was as if the powers of two worlds were instinctively understanding and merging with one another.
Due to the "home field advantage," the powers of his fruits were slowly transforming into the sensation of "Jutsu." If given more time, these two powers might truly turn into a literal Kekkei Genkai.
There were pros and cons to this change.
The downside was that Diarmuid had begun to feel his chakra being consumed when using these powers. If the transformation completed, he might only be able to use his fruit abilities by expending chakra.
But the upside was massive: Diarmuid's total chakra reserves seemed to be expanding alongside this transformation! The Gravity-Gravity Fruit was one thing, being a Paramecia-type, but the Dark-Dark Fruit felt bottomless. Every second, it was converting dense darkness into chakra, flooding Diarmuid's body.
Initially, Diarmuid struggled to cast a single C-rank jutsu. Now, he could toss them around like they were free.
Diarmuid even suspected that if the Dark-Dark Fruit fully converted into chakra-based energy, his reserves might become more terrifying and vast than a Tailed Beast, perhaps even infinite.
Moreover, if the Devil Fruit abilities were naturally converted into Kekkei Genkai by the laws of this world, they would become hereditary. While Diarmuid had no children yet and no plans to leave a bloodline in the Naruto world, you never knew when that might come in handy.
"Hahaha, your Earth Grudge Fear is just as illogical," Diarmuid said with a smile. "I've only mastered three nature transformations, while you just have to swap hearts to get whatever you want."
Of course, he wasn't truly envious. He believed he could eventually develop all five elemental natures. Yin-Yang Release might be difficult, as that required a specific catalyst, but the others were within reach.
Kakuzu looked at Diarmuid, his heart filled with complex emotions. In just six months, Diarmuid had grown to this level. To call him a "genius" felt like an understatement.
As a veteran of the shinobi profession, Kakuzu knew that in the beginning, Diarmuid's understanding of ninjutsu was purely theoretical. He knew nothing of the fundamentals, the details, or the true knowledge.
Though his body possessed immense vitality, his chakra hadn't been worth a damn!
And yet, from the moment he condensed his first wisp of chakra, the guy had been playing with "hacks." His comprehension was staggering; he could take a single concept and apply it across various types of ninjutsu.
One could explain that with intelligence. But his chakra volume was simply offensive.
At first, it was nothing special, but soon it became a bottomless well. Now, in terms of pure chakra volume, Kakuzu felt even he couldn't compete!
What is he? How does he convert internal energy into chakra with such terrifying efficiency?
It defied science.
Then there was that bizarre Kekkei Genkai; manipulating gravity and darkness with such versatility.
Thinking of this, a complex look flashed in Kakuzu's eyes. He thought to himself: In terms of comprehensive strength, he might have already surpassed me... and even the masters of the Five Great Hidden Villages. Sigh.
Decades of life felt like they had been lived in vain.
However, Kakuzu quickly shook off the feeling and cheered up. Diarmuid treated him well and paid him generously. Furthermore, Kakuzu had even learned a bit of that mysterious Haki.
He couldn't use it yet, but he could clearly feel a brand-new type of "Ki" within his body. It was Haki, though still too weak to be wielded as freely as Diarmuid did.
"You... you're a total monster," Kakuzu suddenly said.
Diarmuid blinked, then grinned. "It's been a long time since I heard that description... Heh."
It had indeed been a long time. Years ago, in the Marines, he had held the title of "Monster."
But once he became an Admiral, it was simply taken for granted.
Could an Admiral be anything but a monster? If you weren't a monster, you wouldn't be fit to be a Marine Admiral in the first place.
[1] Abyssal Release (暗遁) if translated literally means Dark Release. However, to distinguish it from the real Dark Release, I chose Abyssal Release.
[2] The reason it says "Dark Release" (暗遁) was plain is because it is indeed plain. Compared that to the chuunibyou-ness of "Abyssal Release," the former sounds plain.