Sure enough, it was the Four Souls Jewel!
Countless people in the real world had the same thought at that moment, because the Four Souls Jewel had appeared long ago.
Aizen had used the one remaining half of a Four Souls Jewel and ultimately relied on the Holy Grail's power to restore it, transforming it into the Hogyoku.
As for where the remaining Four Souls Jewel was, no one knew.
However, everything Aizen had done had made the Four Souls Jewel, already mysterious as it was, seem filled with unknown and immense power.
Without question, the Hogyoku Aizen had paid such a great price to create was no simple thing. At the very least, it was certainly more powerful than the Holy Grail created by the Fuyuki City Holy Grail War.
Otherwise, given Aizen's character, he could have simply pursued the Holy Grail. Why go to such lengths to create the Hogyoku?
And as the foundation, core, and origin of the Hogyoku, the power possessed by the Four Souls Jewel was certainly not to be underestimated.
A single fragment had actually restored Madara Uchiha, who had aged to such a state, to his youth. One could well imagine just how much power it contained.
To be honest, countless people now craved it and wanted it, just as they had when they first learned of the Holy Grail's authority.
Within the Light Screen, Madara Uchiha stared at the Four Souls Jewel fragment, no thicker than a baby's little finger, and asked in a deep voice, "The Four Souls Jewel? What is that?"
Naraku said, "A very special jewel. It possesses immense power, as well as the power to grant wishes."
"Likewise, it was something that should have been completely destroyed. However, someone awakened the origin of the Four Souls Jewel