A/N: This chapter has been rewritten as of February 2019
Prologue:
Minato and Kushina hung limp, impaled by the tail of the Kyuubi as Minato began the hand-seals for the Reaper Death Seal.
"Minato, you self-sacrificing idiot, we'll do it together! We've only been married a year, you think I'm putting up with this 'Till death do us part' shit?! You're taking me with you, you hear me!"
"Kushina…"
"Don't 'Kushina' me, you're not leaving me to the afterlife so you can go be the Death-God's lunch!"
"I…ok. We'll do it together."
In unison, they cried out as they painted seals in their own blood: "Eight Trigrams Sealing Style! 64 Trigrams: Purifying Rebirth."
The Kyuubi screamed as twin divinities appeared: the Shinigami, face pale, took a blade to the beast's soul, devouring each piece as it came free. The Kami, in turn, shone with golden light as a brilliant flame consumed the chakra that formed the body. The bodies of every person in the clearing shone with the same light before they dissolved and converged on Naruto.
BREAKBREAKBREAKBREAKBREAK
"I can't believe that worked!" Kushina exclaimed, her verbal tic rearing its head as she leapt up to hug her husband.
"Neither can I…" Minato sagged with relief, and under the unexpected affectionate assault from his no-longer-pregnant wife, before straightening up in surprise. "Kushina."
"Yes?"
"For the Shinigami's stomach, I expected there to be less damp stone walls and more eternal combat with our mortal enemy."
"…ya know—" she began.
"No, no, NO, NO, NO, NO!"
An echoing, vehement shout interrupted, interspersed with regular, metallic bangs.
"This is not."
BANG.
"How this."
BANG.
"Was supposed."
BANG. "
To go!" The banging stopped for a moment
"They were supposed to die!"
Minato and Kushina snapped their heads around at that. Running down the hall after the reverberations of the voice, Minato stopped short upon seeing the speaker.
"Obito…?" he breathed.
"You bastard, what d'ya want with my son, eh?" Kushina flashed across the room, grabbing the now-named Obito by the collar, interrupting the banging of his head against his cell bars.
"Oh, so we meet again, Hello, Minato. You don't seem surprised to see me." The scarred man scowled, glaring at his former teacher.
Tsunade? Minato mused. No.
He murmured as he thought "…then perhaps, Izanagi?"
Obito's one good eye widened with shock. "Did you just say—"
Minato continued to ignore him, the rest of his musings silent as Kushina put Obito in a headlock.
"Don't. Ignore. Me. What did you want with the Kyubi? And actually, what was the big idea barging in and taking my son as a hostage? You better have a good explanation for this, or your resurrection is gonna be awfully short! You nearly got him killed! …matter of fact, you did get us killed, you bastard!"
Obito choked weakly "Blurgh…"
"Talk, dammit!" Kushina ordered, tightening her hold.
"Uh…Kushina?" Minato said, hesitating as Obito's face began to purple from oxygen deprivation.
She pinned him with a fierce glare. "What?"
"He can't talk when you're crushing his throat."
"Oh…"
Loosening her grip, she grabbed Obito by the scruff of his collar as he struggled to get away. An outside observer would find it amusing to see the S-rank criminal wriggling like a disobedient puppy as he regained his breath.
Realizing the futility of fighting, he sagged in her grip. "It's over, anyway. I've failed."
"Failed to do what?" Minato prodded.
Over the next few hours, Obito explained the Eye of the Moon Plan, and Naruto slept, so exhausted by containing the most powerful entities in the world that he couldn't even muster the energy to cry inside his own mindscape.
-BREAK-
Outside the mindscape, the seals on Naruto's still body thrummed with the power they contained, glowing a faint yellow-orange in the shadows cast by guttering torchlight. Eight seals centered on each of the Eight Gates and flowed up his left leg and crossed his torso, then split at his throat to trace up his temples into a pair of tight Uzumaki spirals on his forehead.
As the Third Hokage, Hiruzen Sarutobi, examined the seals, a growing awe filled him. The strongest seal he'd seen was Minato Namikaze's variant of the Eight-Trigrams Seal, a reverse-engineered work of the Uzumaki style that even in death adorned Kushina's body. The seal marring Naruto's body, was a masterpiece beyond compare, as far beyond Kushina's seal as that seal was from an explosive tag.
Beyond the visible seals Hiruzen could see, visible only to the Byakugan and dojutsu like it, a network of infinitesimal strands etched over the child's entire body channeled as much power as the average Chunin. None was the Kyubi's—Rather, a mixture of the child's own incredible, untainted reserves, Minato's dense, efficient chakra, Kushina's energetic flares of life force, and another, darker energy coursed through the network.
These details went unnoticed by Hiruzen's mortal eyes as the God of Shinobi interpreted the fragments he saw in the seal-work itself as the ink faded, sinking below the surface to lie quiet until called upon.
This section here…a healing seal supplied by output from the containing seal… Reinforcements and dilations to the seal to allow it to channel a truly monstrous quantity of chakra. And, of course, releases at the heart of each spiral to allow the excess to flow into the boy's own pathways. The regeneration rate alone should rival Tsunade's, but the overflow will make chakra control virtually impossible without augmentation. Not to mention the genetic degradation of constant regeneration…
Minato and Kushina made their son virtually indestructible—but at what cost…