History of the Gotei Thirteen

The Founding

Some 2,200 years ago, long before the founding of the Gotei Thirteen, Genryūsai Shigekuni Yamamoto began the Genji School to train worthy students in swordsmanship. At the time there was no official military in Soul Society; instead, most of the Seireitei's might rested directly in the hands of the Great Noble Houses, who were also in charge of handling the purification of souls, with the Rukongai left to the wolves.

Eventually Yamamoto decided he had had enough of the Rukongai being a criminal's paradise, with life brutal for those living within it, so he sought to found an organization that would crush Soul Society's enemies: Court Guard Squads capable of guarding the Seireitei and the Rukongai alike. To do this, he killed two birds with one stone. He swept across the Rukongai, seeking out the most dangerous criminals Soul Society could offer, to whom he offered a simple choice: Join, or die. Though many resisted, with each successful enlistment it became harder and harder to refuse.

This first group made up the original Captains of the Gotei Thirteen, including such infamous figures as Yachiru Unohana, Danjirou Obana, Kanae Nagoshi, and eight others. With the might of these thirteen, the Gotei quickly solidified itself as the mightiest fighting force in Soul Society, dwarfing that of any individual Noble House, and took the authority needed to guard the Seireitei and the Soul Cycle alike. At the time, the Gotei Thirteen was seen as a truly brutal organization consisting of merciless criminals who's reputations alone could strike fear into the burliest of bandits.

The War

On Earth, one of the Five Quincy Pillars, better known as Yhwach, had built an army to wrest control over the Soul Cycle from Soul Society and conquer his enemies. With this army called Lichtreich he invaded, striking the Seireitei in a sudden blitz that swiftly reduced much of the gargantuan city to rubble. Such an invasion was virtually unprecedented and left many in shock, but the Gotei Thirteen was there to defend itself.

Though Yhwach commanded an army of capable soldiers led by personally selected Star Knights, the tide turned the moment the Captains stepped onto the battlefield. With Yamamoto at the fore, they unceremoniously slaughtered all Quincies who intruded upon their home and when the two sides' leaders battled, the Gotei Thirteen was shown to be victorious. Yhwach was dead, and his army a pile of corpses. But the Gotei's mission was not done.

Seeing any Quincies as a threat to Soul Society, the Gotei Thirteen enacted a brutal, horrific genocide against them that lasted years and resulted in their population being reduced to an insignificant number. Even those who had not sided with the Lichtreich were hunted down and butchered.

The Aftermath

After the first Quincy Genocide, Yamamoto and most of his Captains realized, in the wake of war, that they had things worth defending, things they held dear, and chose to usher in an era of peace and justice. Ensuing generations of the Gotei became less barbaric than those who came before, and eventually the organization resembled the Gotei of a hundred years ago.

Though by and large the Gotei did become far less violent, eventually they came to a crossroads once again with the Quincy people. The latter's killing of Hollows had begun to add up, causing the Shinigami to claim the Soul Cycle would be irreparably harmed by their actions. After failed peace talks wherein the Gotei Thirteen and Central 46 demanded all Quincies cease killing Hollows, ignoring their fears this would leave them defenseless, they chose to wipe them out for good through a second brutal genocidal campaign that saw almost the entire race of people to be eliminated to the point of near total extinction. This would mark the final campaign of violence in the Gotei's history until modern times as the Shinigami settled back into their job of handling the Soul Cycle.