Extra Vows
Section 1: Soul Society/Shinigami
Soul Society as a whole predates all known records. Even Head Captain Yamamoto, the oldest living (for a given value of the term) soul in Soul Society, doesn't know Soul Society's age, despite his memories going back some 5,000 years. What is known is that until approximately 2,400 years ago, Soul Society was a lawless warzone, the likes of which make Zaraki look like a paradise. This warzone was split into multiple territories ruled by warlords who ruled with an iron fist. Yamamoto challenged each warlord in turn, the loser having to serve the winner loyally. In this manner, Yamamoto conquered Soul Society as a whole, bringing even the likes of "High Priest" Hyōsube Ichibē and "Kenpachi" Unohana Yachiru under his banner.
With the ten strongest souls and their factions following his command, Yamamoto sought to bring law and order to Soul Society. Over the next four centuries, they established both the Seireitei and Rukongai. Yamamoto in particular founded the Shinigami Academy to train new potential recruits. Despite what some claim, the Central 46 wasn't established for another thousand years. After Yamamoto had to be talked down by his two former students, Kyōraku Shunsui and Ukitake Jūshirō, from exterminating all of the Quincy, including children and the elderly, he decided he could no longer be trusted with absolute power. Not wanting any one person to hold such power, the various captains picked a total of 46 souls to serve as the judiciary branch of Soul Society's government, with a specific law that no members of nobility could be chosen to avoid conflicts of interest. (Later scholars would agree that while a good idea on paper, it merely resulted in 46 corrupt rulers rather than merely one.)
The Gotei 13 was formed by dividing up the current Shinigami forces between the thirteen strongest members: Yamamoto, his ten original followers, and his two best students. Each division has between two thousand and twenty-five hundred members and is led by a captain and their lieutenant. Under them are the third through twentieth seats which constitute the officers, followed by the rank-and-file members. While a division can have multiple officers for each rank up to Tenth Seat, only exceptional circumstances (such as Ukitake Jūshirō's poor health) allow for multiple Shinigami to hold any post Ninth Seat and above. Seated officers in 16th through 20th seats generally lead a team of Shinigami out on missions, while 11th through 15th coordinate the teams their subordinates lead. From 10th Seat onwards, officer positions become increasingly administrative. By the time one reaches 6th Seat, they might well go years without ever having an assignment in the field. While lieutenant rank is sought by overachievers and the role of captain is the goal of dreamers, many ambitious types seeking a "cushy desk job" simply try to reach 6th seat or higher. Due to the vast area Soul Society is responsible for, at any given time approximately half of all Shinigami are either patrolling Rukongai or the Living World.
Though officially separate yet equal, both the Kidō Corps and Stealth Force are generally looked down on by the average Shinigami. The former are seen as failures who couldn't survive real combat (Kurosaki Ichigo would like to stab anyone who thinks this) while the latter are seen as synonymous with the Second Division.
Section 2: Quincy
The Quincy first appeared approximately 1,700 years ago in what was then known as Germania (specifically, eastern Germany, Czech Republic, and western Poland) as a relatively small group of spiritually aware humans who discovered a method for killing the Hollows that preyed upon them. They quickly took to teaching their methods to anyone they found who could learn them. At their height, they numbered at nearly two hundred thousand Quincy in total. Due to the sheer number of souls that were being obliterated, the Shinigami of the time attempted to stop the Quincy from using their powers. But, in a move even Head Captain Yamamoto admits was extremely foolish, they tried preventing the Quincy from ever using their powers instead of only using their powers when actively being attacked. Understandably, the Quincy refused outright, and such an initial attitude caused them to refuse any later attempts at negotiation. As such, Head Captain Yamamoto ordered the Quincy exterminated for the sake of the three worlds. The Quincy fought back, led by their leader, the self-styled emperor Yhwach. While the Quincy vastly outnumbered the Shinigami, Yamamoto had only brought those ranked 5th Seat and above, all of whom were far stronger than the average Quincy. Though a few Shinigami fell, all of the Quincy who fought perished, including Yhwach who was incinerated along with his elite guard by Head Captain Yamamoto. In the end, it was Captains Kyōraku and Ukitake who argued for those too young to use their powers to be spared.
Roughly seven hundred years later, a faction of the Quincy felt their numbers enough to take their revenge on the Shinigami who'd slaughtered them and began hunting and killing any Shinigami they could find. Though ordered by the Central 46 to wipe out the Quincy for good, Yamamoto had grown more tempered in his old age and instead went personally, wiping out the offending faction alone. The remaining Quincy, numbering only a few thousand, were left alone. Unknown to all but a select few in the 12th Division, Kurotsuchi Mayuri has spent decades capturing and experimenting on strayQuincy.
Among the Quincy, there are a few specialized techniques only a few ever access, such as Ransōtengai, Schrift, Vollständig, and Letzt Stil. Ransōtengai was developed to allow the injured or elderly to keep fighting by using strings of Reishi to manipulate their bodies like puppets. Schrift is a power that's unique to each Quincy who unlocks it, often compared to a Shinigami's Zanpakutō. One could gain inhuman speed or the ability to conjure flames or any other imaginable power. Vollständig and Letzt Stil are the two most powerful Quincy techniques of all, granting a Quincy not only monstrous strength, but also complete dominion over all nearby Reishi. The power boost is even greater in Letzt Stil than in Vollständig, but it has the side effect of completely burning out a Quincy's powers in a matter of minutes. In contrast, the only drawback to Vollständig is the stamina drain, which keeps Quincy from using it continuously.
There's a prophecy stating that Yhwach would return after nearly a thousand years. "After 900 years, the Sealed King will regain his heartbeat. After another 90 years, the Sealed King will regain his reason. After 9 more years, the Sealed King will regain his power. Then, after 9 days, the Sealed King will regain the world." The four captains who remember Yhwach all find said prophecy ludicrous. Due to his incredible power, greater than even most captains, Yhwach's soul has been carefully tracked for the last thousand years and has reincarnated several times. He's currently an Acoustic Engineer in Uruguay.
Section 3: Hueco Mundo/Hollows
So far as most know, Hueco Mundo is divided into two parts: The Menos Forest and the Endless Desert above it, though the desert is notably doted by the occasional cave or mountain. The Menos Forest is so named for both the numerous Gillians that inhabit it and the massive tree like crystals jutting straight up to the ceiling. Here is where most lower-level Hollows can be found. The caves leading out are too small for most Gillians to fit through and the Endless Desert is filled with Hollows too powerful for anything that isn't a Menos to survive for long. The sole exception is the few Hollows too small and too weak to be noticed by the Adjuchas that prowl the desert, seeking to evolve into a Vasto Lorde.
An average Hollow usually only consumes a few souls before either being purified or consumed by another Hollow. Most that survive long enough to become truly intelligent, such as Grand Fisher, seek to evolve further. While a Hollow only needs to obtain sufficient levels of Reiryoku to evolve, consuming Pluses is extremely inefficient and hunting Shinigami is extremely dangerous. As such, most instead congregate in the Menos Forest and consume each other. After one Hollow has consumed the equivalent of roughly one hundred Hollows, they will evolve into a Gillian, the largest but lowliest of the Menos Grande. Should one of the souls making up a Gillian possess enough willpower, they will take over the form, giving the Gillian a unique mask and intellect. Said Gillians consume others in order to evolve further. Though exact numbers aren't known, it's believed consuming the equivalent of a thousand Gillians will cause one to evolve into an Adjuchas. Smaller, smarter, and most importantly, stronger, the Adjuchas almost always quickly leave the Menos Forest behind for the Endless Desert.
The Endless Desert is shrouded in perpetual night, with only the waxing and waning of the moon to tell the passage of time. Here, thousands of Adjuchas hunt and consume each other in an effort to evolve further into Vasto Lorde, or at the very least, prevent their decline back into Gillians. While it's unknown how many Adjuchas it takes to make a single Vasto Lorde, some have consumed thousands of fellow Adjuchas without evolving. Currently, there's only five confirmed Vasto Lordes in all of Hueco Mundo, though rumors of more persist. Of the five known Vasto Lordes, the most active and oldest by far is Baraggan Louisenbairn, the self-styled God King of Hueco Mundo. Unlike the other Vasto Lordes who prefer solitude or the company of a few, Baraggan has amassed a sizeable army of Adjuchas, seemingly for no reason than his own ego. Baraggan has attempted more than once to bring the other Vasto Lordes under his rule, but their power along with his refusal to dirty his hands has allowed them to maintain their freedom.
While some Hollows speak in hushed tones of an evolution beyond Vasto Lorde (often citing an old legend of one such Hollow that slumbers under the desert), those who know better remark that there simply aren't enough souls in existence for such a thing to be possible.