Somewhere in the wild mountains, two figures moved through the trees.
It wasn't a real fight. The rhythm was too deliberate, the pauses too considered. This was a training exchange, and both of them knew it.
Finn brought his hands together, ran through the seals, and opened his mouth. A massive fireball rolled out and cut through the mountain air.
"Fire Style: Great Fireball Technique!"
The Great Fireball was Uchiha work, originally. The clan had developed it during the Warring States period, back when the major bloodline families were still fighting each other for territorial control rather than consolidating inside a village structure.
It wasn't a complicated technique; it required no blood limit, no kekkei genkai, no clan-specific coordination. That simplicity was precisely why it had spread. Over generations, it had migrated out of the Uchiha's exclusive arsenal and into the general curriculum. Any shinobi with fire-attribute chakra learned it eventually.
The fact that one of the most recognizable fire techniques in the world was originally Uchiha property said something specific about how much of what the broader ninja world considered "standard" had started in the hands of the wealthy clans.
What those clans had spread were the techniques they didn't mind sharing. The ones they had kept private were different in kind entirely.
Kakuzu watched the fireball approach without urgency. He pressed his fists together in a single abbreviated seal at his chest, and the Earth Grudge Fear did the rest: the technique pulled chakra directly from his stored hearts and shaped it without the full seal sequences ordinary shinobi required. He opened his mouth.
"Water Style: Water Wave."
A dense surge of water met the fireball head-on. Water over Fire. The fireball died in a hiss of steam and scattered vapor.
Then the ground moved.
Before the steam had fully cleared, the soil beneath Kakuzu's feet began to shift and surge upward, folding inward like something trying to swallow him whole. He read it fast. His foot pushed off the rising earth and he flipped clear, landing on a tree trunk fifteen meters away.
He was already scanning the space below when a darkness rose out of the shadow at his feet, tendrils of it reaching upward to close around his ankles.
"Hey." He hopped to another branch and turned to find Finn with an expression of mild amusement. "That's not ninjutsu. We agreed: none of your weird bloodline limit tricks."
"Water Style: Water Breaking Wave!"
Finn had already moved through another seal sequence. A high-pressure line of water shot from his mouth toward the branch Kakuzu had just vacated. Kakuzu used a body-flicker and reappeared several meters laterally, then straightened and pressed a Tiger Seal with visible irritation.
"Fine. You want fire?"
What followed was not a fireball. It was considerably larger than a fireball. Kakuzu released it with a sharp exhale, and for a moment the visible world between the two of them was replaced by flame. A full sea of it, rolling outward across the ground and through the air between the trees, engulfing the water Finn had just fired and the ground it had struck, the heat washing across the whole clearing in a single sustained surge.
Finn raised one arm and extended it toward the fire.
"Dark Water."
The darkness came out of his arm like something alive. It spread across the sea of fire the way a tide covers a shoreline, immediate and total, consuming everything it touched. Three breaths. Four. The fire was gone. Where the entire visible area had been lit a moment ago, there was now a stretch of charred earth and quiet air, the only evidence anything had happened at all.
Kakuzu looked at the scorched ground for a moment.
"Your bloodline limit," he said flatly, "is completely unreasonable."
It had been just over six months since that first conversation in the vault.
Kakuzu had not taken any other contracts in that time. He had stayed in Bear Country, close to the Exchange, and taught. Five attribute ninjutsu, one type at a time, building from fundamentals: how chakra moved through the body's network, how each nature interacted with the others, what the hand seal sequences were and why each one corresponded to the shape of the technique it produced. The theoretical structure, then the practical application, then the refinement until it ran automatically.
He had also, in the opposite direction, been on the receiving end of Finn's Haki instruction. The experience was deeply strange in a way he was still processing. He could feel something new in his body, something that did not correspond to any of the energy systems he had worked with across fifty years of practice. It was faint. He couldn't direct it yet. But it was there, unmistakably, and the fact of its existence told him the instruction was genuine and not theater.
What he had not anticipated was the pace at which Finn was moving.
Kakuzu had taught shinobi before, in informal arrangements over the decades, when the money was right. He had a calibrated sense of what different talent levels looked like at the beginning stages. Finn had started at the absolute bottom: no chakra, no fundamental understanding of how any of the techniques worked at a mechanical level, no instilled habits from childhood training to build on. The theoretical knowledge he carried was academic. He understood ninjutsu in the way someone who had read extensively about swimming understood swimming before getting into water for the first time.
That beginning phase had been slow, as expected. Getting the first strand of chakra to condense and hold stable had taken longer than it would have for a child with prior instruction. That was normal.
After that, it had stopped being normal.
Once the foundation was set, Finn had accelerated in a way that Kakuzu had no clean explanation for. The conceptual grasp was part of it: he could take a technique Kakuzu had demonstrated once, identify what it was doing at a structural level, and apply that understanding to adjacent techniques without requiring the same repetition a typical student needed. That was exceptional intelligence, and Kakuzu could account for it.
What he could not account for was the chakra volume.
At the beginning, there had been nothing. Then a small amount had appeared, condensed from whatever process Finn was using internally. That should have been the starting point: a modest reserve that grew through training over months and years. Instead, the reserve had expanded at a rate that made no sense against any timeline Kakuzu had observed in his career. It kept growing. Not through any technique Kakuzu had taught him. The growth was coming from somewhere inside Finn's own system, something converting the energy already present in his body into chakra with an efficiency that Kakuzu found genuinely unsettling when he thought about it too directly.
He thought he understood the mechanism, in rough terms. The Dark-Dark Fruit, whatever it actually was regardless of how Finn framed it as a bloodline limit, seemed to function as a constant draw on the surrounding dark elements. At every moment, it was pulling that energy inward and processing it. As the two power systems, the one Finn had arrived with and the one this world ran on, slowly worked out their incompatibilities, a portion of that constant intake was being converted into chakra and added to Finn's reserves. The conversion rate was staggering. At the current pace, his total chakra volume was approaching something Kakuzu associated with Tailed Beast territory.
That was not supposed to be possible for a person without a bijuu contract.
In six months, Finn had achieved working command of Fire Style, Water Style, and Wind Style. C-rank techniques were essentially free for him now; he could run them without apparent effort. B-rank required focus but was within reach. A-rank, something like the Great Waterfall Technique, still demanded careful study and didn't come easily, but the ceiling was clearly there to be cleared in time.
There had been one surprise that apparently caught Finn off guard as well. When he had first successfully produced a Water Style technique, he had stopped and looked at his own hands as though he hadn't expected the result. He'd explained, privately, that in his home world, his bloodline limit had made him vulnerable to water: unable to swim, weakened by immersion. He had come to this world carrying that weakness and had braced to deal with it permanently. And then, without any deliberate intervention on either of their parts, it had simply resolved itself. The prohibition was gone. He could swim. He could produce Water Style without any of the resistance he had expected.
Kakuzu's working theory was the same one Finn had arrived at: the world was assimilating him. Not aggressively, not in a way that damaged what was already there, but steadily, filling in incompatibilities and eliminating artifacts that had no anchor point in this world's natural logic. The curse had no equivalent here, so the world had quietly removed it.
The assimilation was visible in other ways too. The suppression Finn had described when he first arrived, the friction on his abilities, had faded considerably. The Haki was still not fully free; Conqueror's specifically remained constrained in ways that made Finn visibly cautious about using it. But the other categories were loosening. And the fruit abilities, the gravity and the darkness, were doing something that Kakuzu watched with real interest: they were developing the texture of jutsu. Not ninjutsu in the formal sense, not yet. But the pattern was there. The power was beginning to interact with the world's chakra framework rather than sitting separately from it.
If the transformation completed, those abilities would require chakra to use. That was the cost. But the benefit was that the boundary between Finn's system and this world's system would dissolve. A power that had been foreign would become native. And given what the Dark-Dark Fruit was already doing to his chakra reserves, Kakuzu was not entirely sure what a fully assimilated version of it would look like. He thought it might be very large.
The possibility that fruit abilities, converted fully into this world's framework, could become heritable as a bloodline limit had also occurred to Kakuzu. He had not raised it with Finn. It seemed like the kind of observation that would either be irrelevant or become very important, and he wasn't sure which yet.
"Ha," Finn said from across the clearing. "Your Earth Grudge is unreasonable too. Five attributes, no seals, pull from stored hearts whenever you want. I can only do three so far." He was grinning. "But I'll get there."
"Yin-Yang Release will be a problem," Kakuzu said. He was not being pessimistic; he was being accurate. "That's a different category entirely. You'd need an opportunity, not just practice."
Finn made a sound that suggested he had already filed that fact away and was not currently worried about it.
Kakuzu looked at him for a moment. The man had arrived six months ago carrying no chakra and no understanding of ninjutsu mechanics. He was now running B-rank techniques at will, holding a chakra reserve that dwarfed Kakuzu's own, and doing it all while also commanding a gravity-and-darkness ability that didn't fit any category in the established ninjutsu classification system. As a student, he was an anomaly. As a case study in development velocity, he was the most aberrant thing Kakuzu had seen in fifty years of operating in this world.
He had lived a long time. He had met a great many people. He had a solid internal scale for where individuals placed in the overall hierarchy of the ninja world.
He looked at Finn and felt the scale shift.
The thought arrived plainly: this man had probably already surpassed him in overall capability. And if the trajectory continued, he would shortly surpass people Kakuzu would not have expected to be surpassable. The Kage tier. The village commanders. People who had spent their entire lives building to that level.
He had spent decades becoming what he was. He had killed more powerful men than most shinobi would ever meet. And six months of watching this particular student had produced, quietly but unmistakably, the feeling that he had somehow lived his entire career pointed at the wrong ceiling.
"You're a monster," Kakuzu said.
Finn paused. Something shifted in his expression, brief and private, before the grin came back.
"Haven't heard that one in a while," he said. "Ha. Long time."
It had been years, in fact. Back in the Marines, the word had followed him for a season, attached to his name the way certain reputations crystallize early and stick. But once he'd made Admiral, the word had stopped. Not because anything had changed, but because at that level, the word lost its meaning. The expectation of monsters was built into the rank. An Admiral who wasn't one wasn't worthy of the position.
If you weren't a monster, what exactly were you doing sitting in that chair?