The world was a stark, black-and-white space created by the Tsukuyomi.
Since it was a mental space, Naruto Uzumaki could manifest the sealing prison from within his body just by wishing for it. A massive water prison materialized, and the spectral form of the Nine-Tails loomed inside. It lifted its head, surveying the area with clear displeasure on its face. "Is this... the Uchiha clan's dōjutsu space?"
"Kurama... long time no see!"
A joyful smile broke through the tears streaming down Naruto's face. This Tailed Beast had been with him his entire life. When he had grown old and frail, the Nine-Tails had desperately tried to extend his lifespan, without a single thought for its own future.
The huge demon fox gripped the bars of the cage and lowered its voice. "Brat, how do you know my name? Who told you! Wait... your spiritual energy..."
This couldn't be right. As far as the Nine-Tails knew, Naruto was just a little brat. Yet here, in the Tsukuyomi space, Naruto's form towered over its own. How was that possible?
"You're much smaller..." Naruto reached out and placed his hand over the cage. "Is it because there's only the Yang Nine-Tails' chakra now? Right, the Yin Nine-Tails is still inside Dad's body..."
The huge demon fox bared its teeth. "Brat, how do you know about this?"
Twelve years ago, when the Nine-Tails went on a rampage, the Fourth Hokage, Minato Namikaze, used the Reaper Death Seal to split it in two. Naruto was left with only half of the Nine-Tails' chakra, the half known as the Yang Nine-Tails. The other half, the Yin Nine-Tails, was taken into the Reaper's stomach along with Minato.
The Nine-Tails considered this a deep shame. But it didn't matter. Even with only half its chakra, it was still the strongest Tailed Beast in the ninja world. Beating up the other Tailed Beasts was still no problem. As long as it could control Naruto...
Naruto extended his fist and pressed it against the cage. "Hey, Kurama, let's do a fist bump!"
"Brat, you even know about that?" The Nine-Tails' eyes narrowed as it watched Naruto. "Do you think I'll choose to believe you?"
This was a secret between the Tailed Beasts and their jinchuriki. Once they fist-bumped, their minds would connect. If their chakras merged, they could even share each other's memories.
"Kurama, don't be so reserved!" Naruto patted the cage and said softly, "Once we fist-bump, you'll know why I've become like this. After we leave here, you can also have me use the Sealing Jutsu: Devour to start breaking the seal by absorbing its chakra."
"Brat, do you think I'll just acknowledge you?" the Nine-Tails snarled. "An idiot like you, whenever you draw on my chakra, you never think about breaking the seal. Why should I believe you?"
That was right. Ever since Naruto first made contact with the sealing space, he only ever wanted to borrow its chakra when he was in danger. After using it, he'd use the Sealing Jutsu: Devour to make the chakra his own. This kind of jinchuriki only knew how to take. A fist bump? Not a chance! If it could ever get out, it would teach him how a jinchuriki should properly serve Lord Nine-Tails!
"Ha, that's because my younger self has forgotten too many things!" Naruto said with some embarrassment. "Sorry, because of my memory loss, I said some very harsh things. I won't let that happen again."
This time, he would absolutely not let the Nine-Tails suffer the fate it did in the future.
"Brat, stop playing tricks..." The Nine-Tails lay down reluctantly and said in a deep voice, "I'm different from the other Tailed Beasts. I will never acknowledge any jinchuriki. Do you think a prisoner would like the cage that imprisons it?"
Naruto was speechless. Are you serious when you say that? After the Fourth Great Ninja War, the other Tailed Beasts all went off to have fun. Only the Nine-Tails and Eight-Tails were still willing to stay with their jinchuriki!
Just as the two were at a stalemate, Itachi's form floated between them. "Naruto, do you need my help?" he said softly. "Within this Tsukuyomi space, everything is under my control..."
As long as Itachi wished, he could not only make the Nine-Tails extend its paw but could even make it do a handstand and spit.
The Nine-Tails' gaze instantly grew vigilant, its eyes locked on Itachi's. "The evil Sharingan again..."
"Let me try one more time..." Naruto scratched his hair and looked down at the Nine-Tails. "Kurama, if our minds connect and you still don't want to be my partner, I won't force you."
The Nine-Tails bared its sharp teeth, its fierce gaze fixed on the huge version of Naruto before it. "Brat, we don't need to connect our minds. I've watched you grow up since you were little. I know your personality inside and out. I know how many times you've pretended to be crazy and dumb just to draw out my chakra..."
"It seems there's really no other way!" Naruto raised a finger and sighed. "Kurama, Shukaku is coming to Konoha soon. It can use its jinchuriki's body to fully transform. Don't you want to bully it?"
"..."
The Nine-Tails' expression froze. Its resolve wavered for a moment before it said coldly, "You're still just trying to use my power! That One-Tail, I can easily defeat it with one finger!"
Despite its words, it was tempted. The Nine-Tails hadn't been out to see the world in a very long time. Recently, it had grown tired of Naruto's repeated antics of pretending to be crazy just to draw out its chakra. Now there was a chance to bully Shukaku...
Anyway, it was just a fist bump. As long as it didn't want to fight alongside Naruto, it wouldn't have to provide the brat with chakra.
"Yeah, yeah, yeah, Kurama, you're the best..." Naruto nodded perfunctorily, clenched his fist, and pressed it against the cage again. This time, he also coated his fist in his own chakra.
The Nine-Tails clenched its paw and looked at him in confusion. "Brat, what do you mean by this?"
"I'm showing you my memories!" Naruto looked at the Nine-Tails with a smile, his voice becoming low and serious. "When we leave the Tsukuyomi space, these memories of mine will disappear. In the future, I might have to rely entirely on you... Kurama!"
The Nine-Tails clenched its paw and extended it, a contemptuous look on its face. "Playing tricks. I know your experiences inside and out..."
Because the Nine-Tails had been sealed in Naruto since birth, it had watched him grow up and knew everything he had been through. Memories... they were irrelevant.
However, when that flood of memories, like a monstrous wave, crashed into the Nine-Tails' brain through Naruto's chakra, it was instantly stunned. Scenes from the future reflected in its mind. Its expression shifted constantly, from bored and dull to shocked, then to joyful and happy. After a long time, a worried look appeared on its face, and finally, only a lingering sadness remained.
It seemed it had reached the end of the memories.
If it hadn't seen them, the Nine-Tails would never have believed that one day it would willingly give up its proud power to extend a human's lifespan. A Tailed Beast that lost a massive amount of chakra... the Nine-Tails knew very well what its fate would be. But if it had to go through it all again... perhaps it would still make the same choice.
The Nine-Tails slowly closed its eyes, and two tears fell from the corners. "Brat... what a long dream..."
"Yeah!" A full smile appeared on Naruto's face. Ignoring the tears welling in his own eyes, he said in a low voice, "So please take care of me in the future, Kurama!"
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Bonus:
Banter between Naruto and Kurama
Kurama: That dream was really nice...
Naruto: Don't get sentimental. There's something you shouldn't forget...
Kurama: What's that?
Naruto: After we get out, my memories will be automatically sealed...
Kurama: What about mine?
Naruto: Yours are fine...
Kurama: So I'll still have to deal with that greedy Naruto?
Naruto: Ahem... theoretically, yes...
Kurama: Starting tomorrow, we're training in the Eight Gates. I have to unseal your memories as soon as possible. If this goes on, I can't stand it!
Naruto: Okay, okay... then don't forget to push me!
Kurama: Oh, right, should we train in Sage Mode?
Naruto: That too.
Kurama: Fine. I'll get familiar with gathering natural energy. I also need to get that brat outside to open the gate first.
Naruto: Don't tell me too much, my head will hurt...
Kurama: So in the end, I have to carry everything alone?
Naruto: Ahem, once I master the Eight Gates...
Kurama: By then, my chakra will have been sucked dry by you