Over the course of hundreds of thousands of years, these Zanpakuto had passed through several masters, built fearsome reputations, each becoming the "strongest" in its own field and leaving behind its own legend.
Later, Nimaiya Ōetsu's forging techniques improved even further. He no longer used powerful souls to forge completed Zanpakuto, instead forging only the semi-finished form—the Asauchi.
These semi-finished Asauchi required Soul Reapers to undergo Blade Zen training themselves, nurturing the "spirit" within the Asauchi with their own souls and allowing it to grow.
Once the spirit within the Asauchi matured, the Asauchi would advance into a "Zanpakuto."
To conceal the darkness behind the birth of the earliest Zanpakuto, Central 46 issued a peculiar order—two identical Zanpakuto were not permitted to exist in Soul Society during the same era.
That was because the Zanpakuto nurtured from Asauchi possessed all manner of strange abilities. No two souls were alike, so naturally, it was impossible for two completely identical Zanpakuto to appear.
However, that first batch of "strongest" Zanpakuto could choose masters suited to them. So long as there was a certain degree of compatibility, those Zanpakuto would respond to a Soul Reaper and become their Zanpakuto.
If two identical Zanpakuto appeared in the same era, the truth that "Zanpakuto are nurtured from a Soul Reaper's soul, born alongside them and perishing alongside them" would be exposed.
To protect the reputation of Nimaiya Ōetsu, the Sword God, the nobles of the Seireitei would never allow such a thing to happen.
If people learned of the existence of this world, Captain Ailin would absolutely have no place in Soul Society.
However...
After looking at the swords in this world, Aizen also knew that such a situation was almost impossible.
Because even without using the Zanpakuto currently held by Soul Reapers of this