Decisive Battle—Divine Age Minato!
The Hokage world was a special world. In this world, there were countless parallel, foreign worlds. Some of these worlds were broken beyond repair, some were utterly barren, and some were highly similar to the real world.
For example, Madara's Rinnegan was connected to one world, and Obito's Kamui space was also very likely a barren world.
Of course, there was another possibility: that both of these worlds had been opened up by their eyes, and had not existed to begin with.
But suppose—
Suppose someone in the Uchiha had once awakened a similar Mangekyo Sharingan and then died?
Perhaps the Uchiha clan had few members, so the possibility was relatively small, but there was no denying that it had always existed!
Holding on to this idea, Shiro began his research. In the end, he not only proved both of these views correct, but also discovered another characteristic—the ease with which worlds could be created!
Simply put, in the Hokage world, it was very easy to create worlds. Of course, they were incomplete worlds, unlike the parallel worlds produced by changes in the timeline. Those parallel worlds were complete, and in a sense, they were real worlds.
But the worlds created through this method in the Hokage world were different. They were incomplete—such as Madara's Limbo: Border Jail, and Obito's Kamui.
Thus, based on this characteristic, Shiro created the great magecraft: Mirror World!
At the same time, he also used the same method to connect to many barren worlds. Unfortunately, he had not been able to find Madara's Limbo: Border Jail or Obito's Kamui.
Originally, with Ayue's ability, she should not have been able to master this great magecraft for the time being, let alone cast it using High-Speed Incantation. But sometimes, talent was just that