The genesis for this fic was the 'Kyuubi as Naruto's Sub and slut' topic in the Restricted Forum on The Fanfiction Network (TFF).
If you are interested in seeing some of the discussions in that topic, as well as the dedicated topic for this fic, a link to the site is at the top of my profile page.
This fic was started after Chapter 519 in the manga. Things will not necessarily be canon compliant.
WARNING: Lemons, 18+, BDSM themes
Taking off his shirt and grabbing a towel, Naruto wiped the sweat off his body, skin gleaming in the midday sun. Looking around at the fauna of the so-called island, he sighed. It was difficult work, trying to use the Nine-tailed Chakra Mode, as he called it. It wasn't just the power that made it hard – it was all the other things that it offered. The raw chakra that made up his shroud could actually be made into immaterial limbs that could be maneuvered, and making it move in reaction to his will was taking a great amount of practice. He also gained a sensor-type ability while in this form, which was far more sensitive than the chakra sensing most sensor-nins used – it allowed him to detect the emotions of all that surrounded him. However, it too would need time to get under total control, as he was currently overwhelmed by the emotions of others around him when using the Kyuubi's chakra.
Speaking of which...Naruto growled. It would be so much easier if the damned fox would just cooperate with him. In his youth, Naruto had to fight tooth and nail to get so much as a sliver of chakra – the fox had allowed him usage of its chakra just once without any drawbacks, when fighting Neji in the Chunin Exams. Every other incident involved Naruto either needing it to survive brush with deaths, when he was emotionally unstable, which Kyuubi used to take control to satisfy its bloodlust, or when his perverted sensei, the late Jiraiya, had loosened the seal.
So he had grown and trained, and just now, Naruto had subjugated the nine-tails for once and for all, with the help of his mother, Uzumaki Kushina, the former Kyuubi jinchuuriki. Even that had come at a cost, as his mother's soul had finally run out of power, and she had faded off to the afterlife.
The blonde grit his teeth. Naruto had promised to himself that he wouldn't let the Kyuubi suffer, and yet for once he wasn't sure about keeping his word. Even now, Kyuubi was draining his chakra every time he used the chakra shroud, preventing him from using shadow clones, from using a Kyuubi transformation like Bee could use to transform into the Hachibi, and even slowing down his regular training. He needed to finish his training, damnit, so he could go out and get into the war that the others were trying to keep him in the dark about.
In the timespan of a few seconds, Naruto made his mind up, as he sat down on the ground, leaning back against a tree. Spacing his breaths out evenly and relaxing his body, he began to meditate. It was...peaceful. With just a few actions, he had let go of his worldly attachments, and felt himself drifting off...
His eyes snapped open, and he found himself looking into slit pupils.
"So you came back, Narutooooooo!"Kyuubi started off disturbingly complacent, but by the end, it had returned to its old mood of mindless raging, roaring out the last syllable of his name.
"Shut up, you damn overgrown fox," Naruto said. Each word was quietly spoken, yet they belied a cool anger that rivalled the Kyuubi's own hot rage. "First, you took my father from me, then you took my mother away. Now, you just won't give up the fight, will you?" Taking a breath, he continued, "In case you haven't noticed, there's an organisation out there trying to kill us jinchuuriki and use you bijuu for their own ends. If Uchiha Madara gets me, that's end-game. After all, as I recall, he can control you with his eyes, can't he?" A soft chuckle escaped his mouth as an idea sprung to his mind. "Unless you like being controlled by itsy-bitsy humans, and just haven't told me yet?"
There was a few second's worth of silence, until Kyuubi blew its metaphorical gasket again. "NARUTO, YOUUUUUU!" It tried to move, but the Shinto shrine posts that Naruto had summoned in his mental realm had pinned the Kyuubi flat on the ground, so it settled for snapping its jaws. "I'LL KILL YOUUUUUUU!"
"Hmph, I'd like to see you try," Naruto taunted the chakra entity. "Besides, fair's fair. You did just end up killing off the last remnant of my mother."
"Actually, not really, Naruto."
Naruto whirled around, and let his jaw drop at the sight before him.
Kushina giggled, "Did you miss me, son?"
The flaxen-haired young man narrowed bright blue eyes, ignoring a barked cry of "Kushina?"from the fox. The whole situation had just gone weird in the span of only a few seconds. He had seen his mother fade away before his eyes, the last of the lifeforce that had fuelled her mental projection having faded away in binding away the Kyuubi and then maintaining a physical form long enough for her to explain the background behind the Kyuubi's attack on Konohagakure. There was no way that this red-haired lady in her mid-twenties standing before him could be his mother.
"I don't know how you managed to get out whatever this is, Kyuubi, but I won't stand for you mocking me. I subdued you before, don't think I can't do it again," Naruto warned as orange pigmentation appeared around his yellow eyes. He didn't think it a wise idea to use the Nine-Tailed Chakra shroud if Kyuubi had somehow managed to manifest a clone of his mother in his own mind, outside of the cage that bound it now.
Kushina snorted. "You-you really think I'm the Kyuubi, don't you? Don't be ridiculous, Naruto." She pointed at the Kyuubi, who had since become silent with Kushina's appearance. "You sealed Kyuubi away again, in something even stronger than what Minato came up with."
"And yet I saw you fade away from this realm with my own eyes," Naruto retorted. "Prove to me you're not Kyuubi."
Kushina took a deep breath, before her head snapped up so fast Naruto thought he heard a crack, and she snapped her fingers. "You're in Sage Mode right now, aren't you, son? That means you can detect my chakra. Give it a try."
Frowning, Naruto wondered what the phantasm meant, but he decided to give it a try anyways. Floating his chakra sense around him, he felt the chakra that made up the form of Kushina. It felt...almost like...
"You're like Kyuubi's chakra is..." Naruto whispered, "Except...opposite? I...what's going on?"
Kushina hmmed, before sitting down on the floor, and pat her hand down. "Come sit with me, Naruto." Slowly, Naruto followed along, still confused about what was going on. "You remember that your father only sealed away half of the Kyuubi's chakra into you, right?"
"Yes," Naruto agreed, wondering where this was heading. "Ero-sensei told me about it while we were on the training trip. Only the Kyuubi's yang chakra was sealed within me, while dad sealed the yin chakra within himself to take with him to the Death God's stomach.
"Indeed," Kushina concurred to Naruto's version of events. "Minato sealed the yin chakra in himself. And then he sealed a part of himself into you."
Gears turned like clockwork in the last living Uzumaki, and he leaped foward from his sitting position with excitement as he realised what the person he now knew to be his mother was getting at. "Oh! You mean-"
"Yes, yes, settle down, son," Kushina berated him, waiting until Naruto had sheepishly seated himself again. "Yang chakra can be thought of as the physical energy that runs through your veins. It's why you have such a ridiculous amount of stamina and chakra, even more than is normal for an Uzumaki. In all the world, only three people might have more chakra than you do, all decades older than you. Given what I managed to see of the outside world in the last few hours, I hope Hoshigaki Kisame was killed in battle today as well."
"And Yin chakra in turn is all about the mind and spirit," Naruto gushed enthusiastically.
"Yes," his mother responded. She thought about reprimanding him for his sudden bounciness, but decided not to. After all, it wasn't like she hadn't been hyperactive at his age. Although it was an odd contrast for him to be in Sage Mode still and still be so wild. "The fragment of Minato's soul that was sealed into your body also held some of the Kyuubi's yin chakra. However, because it was Kyuubi's yin chakra, he couldn't use it. When he faded away after having redone your seal, the yin chakra wandered around in your body. It didn't attack you, since there were safeguards against that, and yet it couldn't reunite with the Kyuubi's yang chakra, either. So when I faded away, it recognised me, and rejuvenated my form."
"M-mom..." Naruto launched himself at Kushina, bowling her over as he hugged her. Tears ran down his blue eyes, the natural chakra in his body having since dissipated. "I-I thought you were gone forever."
"Now, now, Naruto-kun," Kushina said as she patted her only child's back, reassuring him as he alternated between sobbing, choking, and hiccupping. "I'm here for you, son. Maybe not forever, but I'll still be here for a long time again for you."
"So, why did you come here anyways, Naruto?" Kushina asked her son, who had since calmed down. She hadn't been able to view the outside world quite like Minato had, only being able to pick up snippets here and there. She had seen Kisame Hoshigaki appear out of that wrapped sword, Samehada, but other than that nothing else. It was only reasonable to assume that the blue-skinned man had been slain if Naruto had not made much of a reaction towards her comment hoping for his death.
"Oh," Naruto grit his teeth as he remembered what he had come for, before he had gotten sidetracked by the appearance of his mother. He looked to the side at the red mass of malice and corrosive chakra that was called the Kyuubi no Yoko, which had been staring at the two throughout their conversation. "You and I sealed the damn fox away again, but it refuses to let me use its chakra without trying to drain away my own, and I can't transform into Kyuubi like the eight-tails host can as long as it continues being hostile."
"Hmm..." Kushina hummed for a bit, tapping her lips, before she made her decision. "I'm pretty sure I can solve your problem, but you'll have to give me permission to do a bit of redesign in here."
"Huh? Uh, sure, ok," Naruto responded. He had an inkling she meant altering this realm that was the juncture of his mind and soul, given that he had created a stronger cage for the Kyuubi without applying any physical changes to the spiral fuinjutsu on his stomach. What she intended to do, he had no idea, but he trusted her. She was his mother.
"Good, good," Kushina nodded, before walking up closer to the cage, staring Kyuubi down despite her relative tininess.
"Kushina..." Kyuubi spoke again, after having remained silent for the last few hours. "What-what are you p-planning on d-doing?"
Naruto did a double-take. Had the Kyuubi just stuttered?
"Why, Kyuubi-chan, you sound so nervous," Kushina just smiled. Naruto recognised the smile for what it was: the mischievous smile of a tricker who had one last big trick up her sleeve. "You don't want to be my uke again?"
"W-WHAT?" Naruto shrieked. What had his mother just said?
"D-don't do this, Kushina-sama, please don't!"The Kyuubi whimpered.
Naruto's jaw dropped. The Kyuubi had just whimpered. The Kyuubi, the mass of malevolent energy that had attacked his home and would have destroyed it if not for the joint efforts of his mother and father, had just whimpered. What. The –
"Change now, Kyuubi-chan," And it was an order, "Or else it'll be the horse again."
The blonde genin of Konoha could only observe as Kushina and the Kyuubi went through a stare-down. Slowly, however, the Kyuubi began to look away, before it...shifted, for the lack of a better word. It started dwindling in size, its form changing from being stretched out horizontally to becoming more height-oriented. Two of its legs retreated into its body, before two more limbs sprouted out of the top of its new shape, becoming humanoid in the process.
The result was a naked girl looking to be in her mid to late teens kneeling on the floor before Kushina, large bare breasts exposed as the new female had her head reared to one side, red hair bordering the color of black touching the ground, exposing her neck for all who were predators to salivate at. Most notable was the single flame-red tail coming out of her body just above her buttocks that was lying against the floor, and two red fox ears that laid flat against her head. "Mistress."
"Ah, your training is finally beginning to kick in," Kushina praised. "I was afraid that I would have to break you again. As it is, you'll be going through punishment again. Trying to grab freedom at the first chance you saw and fatally wounding me in the process, stabbing me through my body with your claws, refusing to make life any easier my son...you've racked up quite a set of punishments for yourself."
"Kyuubi will accept what mistress decides," The fox-turned-girl said, bowing her body down until her head touched the floor.
"Ah, but it won't be me who decides on what to put you through for your insubordination," Kushina replied, as she summoned up a plain, black leather collar with a leash attached to a D-ring on the front. "You decided to go and get yourself sealed in my son, you'll have to deal with the consequences. Naruto-kun, come here and collar your new slave, will you please?"
"Huh?" Kushina turned around, and stared at her son. Surprisingly, he was still standing, but he looked like he might fall over at any second, with the dazed look on his face and glazed eyes. "Oh dear," She giggled, "It looks like I broke him." Looking back at the one-tailed girl, she frowned. "Oh well, when my son recovers, I'll get him to put this collar on you. Won't that be fun, my sweet little uke? Instead of me having to make do with all my toys, you'll finally get the real thing penetrating you." She said all this as she stroked the red-black hair of the being whom she had broken and dominated almost two decades ago.
No, there isn't going to be any incest. No, there isn't going to be one of those craptastic "Well Naruto's DNA changes because of the Kyuubi's chakra so it's not technically incest!" excuses I see other authors bandy about.