"The day would come when every enemy had been slain. After the Uzumaki Clan eliminated the last ninja clan that had oppressed them, they found themselves in a desperate predicament.
Because of their sense of morality, and to avoid becoming everyone's target through unnecessary killing, the Uzumaki clansmen of that time chose to go into hiding again.
But with less killing, they could no longer satisfy the Shinigami's demands. At first, things were fine. Perhaps because some affection still remained between them, the Shinigami showed no strong reaction.
However... everything changed one day when a Shinigami Jinchūriki suddenly went berserk. The other Shinigami Jinchūriki could no longer draw on the Shinigami's power, while the berserk one slaughtered people within the Uzumaki Clan.
The bloodshed only ended when a clansman proficient in sealing jutsu stepped forward and completely trapped the berserk Shinigami Jinchūriki.
This incident also made the Uzumaki clansmen of that time realize that the price of the Shinigami's power... was far too heavy. But there was one good thing about it.
They discovered that their sealing jutsu could curb these Shinigami's rampages to a certain extent. From then on, the Uzumaki Clan began taking precautions against the Shinigami.
Seals were placed on almost every Shinigami Jinchūriki, and indeed, no more rampages occurred after that.
But, as I said before... the Shinigami would pass on to new hosts, with no pattern whatsoever. The only rule they knew was that, after the previous host died, the Shinigami would suddenly appear inside a child with the Uzumaki bloodline.
And with every succession, the Shinigami's power grew stronger. So the Uzumaki clansmen of that time began desperately researching all kinds of sealing jutsu, hoping to develop a seal that could stop the Shinigami from passing on altogether. Unfortunately, they only half succeeded.
After