The kyubi was staring at the 4th Hokage in horror as he summoned the shinigami since he knew that not even he the strongest demon in existence could stand up to a god.
When kyubi began to get dragged into the seal on the baby that insolent little ape wanted to seal him in, he however somehow knew that the shinigamis power was currently limited by the amount of chakra the Hokage had.
This revelation gave the kyubi hope that he might escape if he struggled enough, because of the increased struggling the hokages chakra ran out when only 8 tails was in the seal and the Hokage was forced to close the seal early cutting of the tail.
If the kyubi had been a normal living being this would have been no problem since the tail would simply have been a dead limb, but the kyubi am a being made out of pure demonic chakra, and it has pushed as much as it chakra as possible into the tail in its struggle this meant that the tail had as much chakra as the sanbi at the moment.
This was enough demonic chakra that instead of dispersing as small amount of demonic chakra normally do when separate from the main body, it began to slowly change its shape and build a new consciousness based on all the negative feelings on the battlefield.
And since this would be a new demon it unlike the kyubi wasn't bound to the other biju in its maximum power but the chakra it would be forming from was indirectly from the jubi since the kyubi once was a part of the jubi this meant that this new demon would have the potential to become a new jubi.
The shinigami couldn't allow that, the last jubi was so grave a threat that kami herself had to step in, and give a warrior so much power that it became a part of him to the point that it became a part of his genes meaning that it became hereditary.
Kami did not like that at all, since even thought his descendants only had a weaker version of his power they still had a bit of kamis power without being chosen by her. So kami would be very mad at the shinigami if he allowed a new raging jubi to be born.
The shinigami could however not do anything to destroy the new demon since he was only allowed to interfere with the living world if he had been summoned, and then his powers was based on his summoners willpower and chakra.
And even thought the 4th at full power would easily have enough power to allow him to destroy the future demon. As it was at the moment however after the sealing of kyubi he only had enough power to seal a human soul, so the shinigami could only see one way to at least make sure the demon wouldn't be a mindless force of destruction.
He forcibly used the last of Minatos chakra to seal Minatos own soul in the demonic chakra, thereby making the chakra stop trying to create a new consciousness since it now had one.
The shinigami then ordered Minato to hide his real identity and act as a guardian spirit for his son. he didn't explain why he just said it was important.
The shinigami then went back to his realm to find kami and explain what happened from his view, and hopefully avoid being punished.
Last time kami punished him he was forced to spend a thousand years working in the afterlife's orphanage for those who died early and he hated children.
But maybe he thought just maybe he will let me go since I gave up my chance to eat Minatos soul in order to limit the chance of a new jubi going on a rampage in the living world she always loved heroes so maybe the fact that this incident gave one of them a semi happy ending will make her forgive me I sure hope so I never want to go thought working at the orphanage again the shinigami thought and shivered.
This is in the naruto universe for now but I might end up making it a crossover later I love crossovers