Kushina screamed.
In her wordless howl was all the rage, all the defiance and fear and unflagging, relentless will that had made her one of the most feared ninja of the Leaf. With that will, made manifest as great, shining chains, she restrained the Beast that would harm the ones she loved, her husband and newborn son. In a way, it was what she had always done, what she was for.
The great fox bellowed, its rage spread out across the land. All across the Elemental Nations, sleepers tossed and turned. Come morning, they would wake from troubled dreams, unable or unwilling to remember the fear that had come upon them in the night.
Miles away, those ninja with an unusual sensitivity to chakra signatures, Sensors, fell to the ground as if seized by some sort of epileptic fit. Within the village below, the fox's hate was a palpable thing. It pressed down upon the inhabitants like some terrible weight. Civilian or shinobi, everyone felt it and was struck down in a moment. Some of their number, especially among the weakest, the elderly, the sick and the infants, would never rise again.
The Nine Tailed Fox was a being composed of power and hate, unique in this world. Thank any god you choose for that.
Minato did not feel fear. Despite the sensation of his wife's flickering chakra, weakening as her life faded, his focus was on the motion of his hands as he finished the technique he had never wanted to use. Despite the wound in his side, deep enough to threaten his survival, a memento from the masked stranger who had unleashed this upon them before he had driven the man off, his focus was on the infant at his feet.
Minato had never wanted this day to come but he had seen its possibility, had created plans and contingencies and in the planning he had decided. Once the course was set there was no time for fear or regret. That was how he had lived his life, with plans and focus and determination, without regret.
It was done.
Some... something answered. It was immense and dark and terrible and none of those things, like the gap between action and result, like... Minato could not understand it. The thing was too much, too alien. He felt a sense of- of acceptance from it. The bargain was set.
A piece of it was thrust out, into and through him, linking them. It hurt, hurt terribly. It struck the Nine Tails and... it struck his son. For a moment, they were all connected. Minato did not feel regret. This had been decided. The feeling of connection twisted and the fox, that thing of hate and terror, was drawn into...
A stream of red, a feeling of power passed through him and into the newborn, taking something from Minato as it passed. It left a mark on his son, an infinitely complex symbol on his stomach. He felt weak, fell to his stomach. There was little time left but enough for one more thing.
There had to be.
Minato reached out, laid his trembling hand upon the head of his son.
Kushina watched, too weak to move.
Unseen, too injured to continue his battle with the Hokage, a man wearing an orange mask watched as well. He had barely enough energy to hold himself apart from the world, safe within the confines of his eye's power. Even if he could have, he would not have intervened. He told himself this again and again.
The Fourth Hokage fell to the ground, dying, his life forfeit to the bargain he had struck. The forbidden technique had sealed the fox within his son, tied their lives together. While one continued, so would the other. When one ended...
There was fresh blood on Minato's hand.
Kushina saw and called for her child as her heart broke. "Naruto!"
The man in the orange masked looked on as his plans shattered but that was not the source of the horror in his voice. "Sensei... What have you done?"
Minato had no regrets. Again and again, he told himself this as he slipped away into the darkness.