- "That is not for you or for me to judge. Because this entire world is established upon a pedestal of sin. But as the forefather of the Quincy once wanted, it will be no good for the world to go back to what it used to be in the beginning. It is crucial that people not lose their significance as a 'person'."
- —Sōsuke Aizen regarding Kaname Tōsen's hatred of the Shinigami.
The Original Sin (原罪, Genzai) of Soul Society is a horrible act perpetrated by the founders of the Five Noble Houses. In the beginning, the old world was one without death, yet existed as a chaotic mingling of both stagnation and progress; constantly circulating between the intertwined life and death. They who would become known as the five noble house founders created the existence known as the Soul King and caused the world to split into its current state, allowing life and death to take on separate forms.
- "The Soul King is a sacrificial lamb. The world did not always have its current form. Without a boundary separating life and death, within this chaotic world... at first, there stood the original guardian; between Hollow and Man. Quincy, Shinigami, Fullbringer... An existence who could be called the progenitor of all three... It was a Quincy as well as a Shinigami, also a human. A symbol of hope governing the chaotic world with innumerable abilities like that of Fullbringers. The three worlds were created by sacrificing a being who was both a devil and a messiah. By five individuals. Whom we now know as the ancestors of the Five Great Noble Houses."
- —Tokinada Tsunayashiro speaking of the original sin.
In the era before the creation of the three worlds — Human World, Soul Society, and Hueco Mundo — all of creation was in a state of ambiguity. Life and death were of one form. Progression and regression flickered to and fro, as it took at least a hundred million years to cool down. Eventually, Hollows became a part of this circulation of souls. Hollows began to devour Humans and the circulation started to slow down. All those souls came together to form a gigantic Hollow that would later be known as a Menos Grande. The world came to a standstill and stagnation reigned. During this time, however, a new life had come into being. As Ichibē Hyōsube calls it, it was as if the world itself was naturally rejecting its own stagnation. This new life form destroyed the Menos and turned it into sands of reishi, allowing the circulation of souls to continue once more.
This individual would become known as the Soul King. Many other beings with strange and fantastical powers were born during this time, but the Soul King was noted as standing out among them all; being said to possess powers bordering on omnipotence and omniscience. As the Soul King continued to protect the world from the Hollows by annihilating their souls, akin to the Quincy that would be born in the millennia that followed, the world itself was beginning to sink into chaos. Among those who did not find this situation agreeable, there were five prominent individuals that would become known as the founders of the Five Noble Houses — the Tsunayashiro, the Kuchiki, the Shihōin, the Shiba, and an unnamed fifth.
Each of the founders had different motives. The Tsunayashiro ancestor was afraid that the Soul King's great power would someday be used against them. The ancestor of the unnamed family said it was necessary to have a world that could serve as a "lid" to cover up the "pit" that would later be known as Hell. The Kuchiki ancestor believed that a new order was necessary to solidify the world and pacify its chaotic state. The Shihōin ancestor said it was necessary to form a larger circulation of souls in order to advance the stagnant world. And the Shiba ancestor wanted to pursue a merciful path in which Hollows could be purified rather than destroyed as they too had hearts. Regardless of the difference in method, the founders had all come to the same conclusion: to split the world. A world of order, a world of implementation, and a paradise of sand for the Hollows. It was noted that other worlds could also come about as well, but the essential aim was the clear separation of life and death. In order to make this a reality, they required the power of a man that transcended everything; namely, the Soul King.
Although the Shiba ancestor was against it and tried to talk the others out of it, the Tsunayashiro ancestor took charge of the plan and it was said the Soul King was tied up and sealed within the crystal where he would lay as the lynchpin for the new worlds. Ichibē Hyōsube would go on to witness the rest of Soul Society's history from this point forward. Using the Soul King's powers of "The Almighty" as the keystone, the five ancestors created the foundation for this new world. Soul Society, the Human World, and Hueco Mundo would be born, and thus, life and death were finally separated. A new era could now be ushered forth. The ones who would manage the newly-born Soul Society would go on to become known as the Shinigami. In Ichibē's words, "Perhaps it was because he could see the far-flung future... Maybe he could not escape his fear, or maybe he found some hope in the new world... I don't really understand... But the Soul King dared not resist it." His words seem to suggest that the Soul King willingly went along with the will of the five ancestors because of his power to see into the far-flung future.
- "Oh, he most certainly had a will of his own. It might have faded with the passage of time, but I think he was aware of the general situation and thus guided Kurosaki Ichigo here to the Reiōkyū. This shows that he possessed his own will. I could feel it. If the bone is the Ōken, then a part of his soul is entrusted to this Reiōkyū."
- —Ichibē Hyōsube speaking of the Soul King.
However, this is where the horrible act that would become known as the 'original sin' was committed. The Tsunayashiro ancestor doubted the Soul King's compliance and feared that the Soul King would decide to escape from his sealings of his own will. He neither wanted the Soul King to live nor did he wish to kill him. In act of this helix of contradictions, the "right arm of stagnation" and the "left arm of progress" were torn off from the Soul King's body. And yet this was still not enough. All five of the ancestors, including the Tsunayashiro ancestor, took part in the violation of the Soul King's body. They gouged out his heart, internal organs, and even dismembered both his legs. In doing so, they managed to destroy his power, creating a King convenient for themselves. The Soul King would serve as a lynchpin of the world without ever having a say in its workings.
- "The Shiba ancestor did rebel against the Tsunayashiro ancestor for sealing the Soul King by force. He tried to find a way to sacrifice himself instead of the Soul King. I had hoped that the sinless would be exempt from the other crimes. In a sense, it was a most foolish thing for him to do. If he had not failed to persuade, then he would have had to prepare himself to be the cornerstone of the three worlds. It's ironic, really. The Shiba bloodline was allowed to thrive only because the Tsunayashiro ancestor decided to seal the Soul King. But when the descendant Ichigo appeared, possessing all the qualities that made him fit to replace the Soul King, I really thought that history was repeating itself. Perhaps the world needs someone like him."
- —Ichibē Hyōsube commenting upon the Shiba clan and Ichigo Kurosaki.
The Shiba ancestor was said to have been the sole opposition to the Tsunayashiro ancestor's plan despite his reluctant participation in the original sin. He tried to find a way to sacrifice himself instead of the Soul King but failed to convince them. However, it is noted that it was because he failed to persuade them that the bloodline of the Shiba clan was allowed to thrive. Both the right arm and left arm were said to eventually find their way back to the Reiōkyū along with their respective wills. While the right arm was enshrined within the Soul Society as its protector, the left arm, as a Quincy under the influence of Yhwach, was meant to restore this world back.
This 'original sin' as it would become known as was inscribed onto a stone tablet that was held by the Tsunayashiro clan for generations before being demolished by Tokinada Tsunayashiro. It would remain a deeply protected secret of the Soul Society, with few throughout its history ever learning of it, and of those who did and remained loyal to the Soul Society feared the original sin ever becoming public knowledge; believing that even the most devout of loyalists would be persuaded to rebel against the Soul Society if they were ever to know the dark secret behind its creation. Although the exact relationship is unknown, the original sin is tied to the existence of the Fullbringers; particularly, it is related to the reason behind why Hollow reiatsu affects a fetus within the mother's body so greatly.