The strength of a Devil Fruit user depended on three factors: Their understanding and development of the Devil Fruit, their own personal strength, and the inherent potential of the Devil Fruit itself.
Take the Mag-Mag Fruit as an example—it was one of the top-tier Logia-type Devil Fruits, possessing the highest offensive power among the three Devil Fruit types.
Even if someone besides Akainu were to eat the Mag-Mag Fruit, the person would still be able to create and manipulate magma, meaning its power wouldn't be weak no matter what.
On the other hand, the Calm-Calm Fruit, no matter how much Rosinante developed it, would never be particularly powerful.
Then there were Sabo and Blackbeard.
Both consumed Logia-type Devil Fruits—Sabo with the Flame-Flame Fruit and Blackbeard with the Dark-Dark Fruit. Not only were they personally strong, but they also had a deep understanding of their abilities. That was why Sabo, immediately after eating the Flame-Flame Fruit, could use it even more skillfully than Ace. And why Blackbeard, right after acquiring the Dark-Dark Fruit, was able to dominate Ace in battle.
As for Tanaka, while he might not be as strong as Sabo or Blackbeard, he had Law as a predecessor who had already done all the groundwork—he just needed to copy it.
Maybe he wouldn't awaken the fruit immediately or activate K-ROOM right away, but the basic abilities of the Op-Op Fruit? As long as he had hands, he could use them. Just a little trial and error, and he'd wield its power with ease.
The only limitations would be the degree of development and his own strength, whether it was the overall potency of the Op-Op Fruit or the size of the ROOM. For now, he couldn't match Law during the raid on Onigashima.
But catching up? That was only a matter of time.
"Thank you all for bringing me the Devil Fruit. I have nothing to repay you with. So I'll send you off with this fruit."
After testing the Op-Op Fruit's abilities, Tanaka stopped pushing its limits. Instead, he picked up the dining knife in his hand.
In the despairing and furious gazes of the pirates before him, he swung it down.
Slash.
A familiar, faint pain came from their necks.
And then, just like their severed hands and feet earlier, the heads of every pirate were cut off, rolling on the ground several times before coming to a stop.
Clearly, a five-billion-berry slash was something they couldn't withstand.
"Monster!"
Their limbs and heads were separated.
Even if the world's strongest man had been there, he would have been dead. Yet the pirates remained alive. The heads rolling on the ground opened their mouths, letting out screams of despair and terror.
At that moment, they finally understood why the World Government was willing to pay so much for the Op-Op Fruit.
It was practically godlike.
And at the same time, that was exactly what Tanaka was thinking.
With ROOM in his hands, he could achieve anything.
But soon, he forcefully suppressed his rising arrogance.
Forget about the Five Kage, Madara, or even stronger foes like Kaguya from the Naruto world.
Even Law, who had awakened the Op-Op Fruit's power in the future, still lost bitterly when facing Blackbeard with two Devil Fruits.
Godlike? Far from it. Whether it was the strongest in the shinobi world or the strongest in the pirate world—right now, he was neither.
However, while he set aside his arrogance, his confidence remained unshaken.
With the Void Walker's Gate, the Op-Op Fruit, the Byakugan, chakra, Haki, and all the techniques of ninjutsu, taijutsu, and genjutsu—By combining the resources and power systems of both the Naruto and One Piece worlds, whether it was becoming the strongest in both worlds or achieving absolute invincibility, he firmly believed that, given enough time, he would stand at the very pinnacle of both worlds.
---
Inside an unremarkable house, Rosinante, draped in his feathered coat, crouched down carefully and placed the unconscious Law onto an old bed. The boy had long since fainted due to the worsening symptoms of Amber Lead Syndrome.
"Law. Just hold on a little longer—we're here."
"I'm going to bring back the Op-Op Fruit right away. Believe me, as soon as you open your eyes, you'll see it."
He looked at Law, who, despite being unconscious, still had snot running from his nose. A trace of pain flickered in his eyes.
Seeing Law's breathing grow weaker and his face even paler, he didn't dare waste any more time. He wrapped the boy tightly in his own sweater, ensuring he was well covered. Only then did he activate the power of the Calm-Calm Fruit, silently opening the door and dashing toward the highest building in the town.
That place was where Barrels and his crew were holed up.
However, by the time Rosinante used his Devil Fruit powers to mask his presence and moved like a ghost through the snow, reaching the pirate stronghold at the mountain's peak.
He froze on the spot.
What greeted him was not a heavily guarded hideout full of pirates, but corpses.
Blood stained the pale snow, and the entire area was covered in craters, as if it had been bombarded by artillery.
"What happened? Did Doflamingo lose patience?"
"Did he come to steal the fruit himself?"
At the sight of such devastation, Rosinante's first thought was of his inherently evil older brother.
The mere idea of Doflamingo's presence sent him into a panic. Without hesitation, he activated the Calm-Calm Fruit again, ensuring no sound escaped, and rushed into the shattered remains of the hideout.
But the moment he stepped inside, the sight he expected was nowhere to be found.
"It's not Doflamingo."
For a brief moment, he felt relief.
Then, as he surveyed the surroundings, his heart turned ice-cold.
Scattered all around were pirates, dismembered.
Normally, if someone's head and limbs were severed, even the so-called "strongest creature" of land, sea, and air would be dead beyond saving.
Yet these pirates, even with their heads detached, they were still alive.
To most, this scene might have been incomprehensible. But Rosinante—once a Celestial Dragon, the adopted son of Fleet Admiral Sengoku, and a high-ranking officer of the Donquixote Pirates—understood immediately, even with just a fleeting glance.
"It's over. The Op-Op Fruit was taken."
A wave of dizziness hit him. His vision darkened, his mind went blank, and he felt as if the world was spinning. He nearly collapsed right then and there.
He had failed.
He had promised Law that he would bring back the Op-Op Fruit, and now—he couldn't.
Despair engulfed him.
Just as he stood on the verge of breaking down, a voice reached his ears from not far away.
"I bet you're thinking: The Op-Op Fruit was stolen."
"Am I right?"
Though phrased as a question, the voice carried no hint of doubt, only certainty.
"You… You ate the Devil Fruit?"
Rosinante followed the voice and saw a black-haired boy, his face still youthful, no older than Law. The boy sat leisurely in a mahogany chair, a knife in one hand and an apple in the other, casually peeling the fruit.
The fury that had been boiling inside Rosinante—the rage at losing the Op-Op Fruit—had been about to explode.
But upon seeing how young this boy was, possibly even younger than Law, half of that anger dissipated.
What remained was not rage, but helplessness and regret.
Not hatred toward the boy in front of him, but hatred toward himself.
For being too slow.
If he had just been a little faster, the Op-Op Fruit wouldn't have been taken.