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Below is a timeline of the history of Yōkai and the Hyakki Yagyō, as well as events of the Arc.

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⋮☰ Contents

1. Pre-Treaty
2. The Treaty
 2.1. Yōkai Side
 2.2. Shinigami Side
3. Wazawai & Shukusei
 3.1. Shukusei's Death
4. The Last Hyakki Yagyō
 4.1. The Aftermath
5. The Jinja Hime Incident
6. The Hyakki Yagyō Returns
 6.1. Mayuri's Research
 6.2. The Hō'ō's Blessing
 6.3. Reiko's Vision
 6.4. Chōshin's Summons
 6.5. Yamamoto's Orders
 6.6. The Race Against Time
7. Fights

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Pre-Treaty

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• The Hyakki Yagyō

Beginning	End
Umiko vs. Issei Shiba	The Yōkai Alliance vs. Shukusei

Place: Seireitei, Soul Society, Hueco Mundo, Karakura Town, & Ashuradō

Combatants

• Kurosaki Clinic

• Karakura Town Residents

• Urahara Shop

• Takonoko

• Midnight Novelty

• Visored

• Rebirth

• Las Noches

• Gotei 13

• Kidō Corps

• Togakushi-yama

• Ashuradō

• Sora no Jinja

• Bount

• Shukusei

Since the beginning, Yōkai have existed both in their realm called Ashuradō as well as the Human World. In the pre-Heian period (?-794), Yōkai were prevalent throughout Japan; although there were benevolent Yōkai, they were more well-known for their mischievous or malevolent nature, especially those that are connected to Oni. Their existence, however, was largely ignored by the formation of both Shinigami and Quincy, as Yōkai have an innate connection to nature, and the existence of Ashuradō placed their life cycles primarily outside that of Human Souls, thus not threatening the delicate balance between Worlds. In the time between the Heian period (794-1185) and Azuchi–Momoyama period (1568-1600), an event known as the Hyakki Yagyō took place every year, led by a Nurarihyon named Chōshin. Yōkai and Oni alike would gather in the streets, wreaking havoc on Humans unfortunate enough to find themselves out as the procession occurred. The Hyakki Yagyō would take place in a different location within Japan every year.

During the end of the Nara period (710-794) and start of the Heian Period, a young Shinigami trained at the Genji School named Issei Shiba travelled to the Human World to patrol his assigned area of modern-day Kan'onji, Kagawa, Japan. while hunting a Hollow, he had his first encounter with a certain Yōkai: The First Emissary of Ryūgū, (the Ashuradō Sea Palace of the Yōkai Dragon Lord, Ryūjin,) the Jinja Hime Umiko. Umiko promptly vanished before he could properly get to know her, but over time the two would develop a rather heated yet ultimately antagonistic relationship between them both. Sometime after the turn of the century (after 800 CE,) however, Umiko began distancing herself from Issei, having grown closer to a local Buddhist monk from the recently formed Kan'nonji temple named Yūma, who happened to be a Quincy. Upon discovering the nature of Umiko and Yūma's connection, Issei grew enraged and attempted to kill Yūma himself, claiming it was more to do with him being a Quincy. Before he could, Umiko stepped in to stop him, taking Issei's right eye in the fight and half-blinding him. Umiko and Yūma then fled the area, vanishing from Kagawa entirely; it wasn't until many years later that they both came up on Soul Society's radar, when it was discovered that Umiko and Yūma had given birth to the first Yōkai-Quincy hybrid.

Between this union and the increasing violence of the yearly Hyakki Yagyō, the Shinigami of Soul Society began planning a potential strike against the Yōkai of the period in order to send a message. Issei himself personally stepped forward to help this cause with the knowledge he'd gained regarding Yōkai given his numerous interactions with Umiko, although he attempted to keep his involvement quiet lest Umiko herself find out. Tensions between Shinigami and Yōkai began to come to a head as the threat of war from both sides began to loom.

The Treaty

Despite Issei Shiba's best attempts, word reached Umiko regarding his contributions to the Shinigami war efforts. Finally emerging from the quiet life she had been attempting to live with her growing family, Umiko and her now-husband Yūma arranged a meeting with Genryūsai Shigekuni Yamamoto in order to discuss a more peaceful alternative to all-out war. Using her status as the First Emissary to Ryūjin, Umiko stated that she would help ensure a minimalization of Human casualties during all future Hyakki Yagyō to avoid the need for Shinigami interference. As both the First Emissary and a prophetic Yōkai, Umiko possessed incredibly strong mediumship talents, includuing the ability to channel Ryūjin in order to enact his will in the Human World.

The meeting, which had initially been made in the spirit of seeking peace, quickly became one of mutually assured destruction: Should Umiko fail in her duty of minimizing Human casualties during future Hyakki Yagyō, then the Shinigami had grounds to attempt to wipe them out- But conversely, should any Shinigami attempt to interfere with or harm Yōkai without legitimate cause, Umiko would channel Ryūjin and bring about far worse ruin. Because of the effective gridlock between the two factions, this lead to the Treaty's creation. Drafted by both Umiko and Yamamoto, and overseen by members of Yamamoto's band, Issei, and Yūma, an agreement was laid out that Shinigami would be forbidden from interfering with Yōkai affairs, just as Yōkai would be forbidden from interfering with Shinigami affairs.

Yōkai Side

For many Yōkai, the Treaty was seen as an insult to their very existence; instead of being viewed as a means to be allowed to continue the Hyakki Yaygō each year, it was instead seen as a threat: To many, it was not a question on if the Shinigami would strike against the Yōkai for simply acting in their nature, but when.

There were, however, some Yōkai who were in favor of the Treaty; just as there were those who delighted in the revelry and chaos of the parade, there were others still who had sought to protect Humans from their own kind, seeing the Treaty as a means of allowing Yōkai who enjoyed the Hyakki Yagyō their fun without killing innocent Humans in the process.

Shinigami Side

On the Shinigami side of things, the Treaty was met with mixed opinions; there were those who thought the Treaty was too weak a balm for a greater wound, essentially viewing it as an open invitation for the Yōkai to cause incredible harm to Humans later on down the line, while others saw it as the best of a bad situation, a means of keeping the balance of the Worlds without risking more lives in the process.Following the signing of the Treaty, Issei Shiba was given the special duty of privately overseeing the yearly Hyakki Yagyō to ensure Yōkai compliance; he did this for many centuries following the signing, even when he himself once served as Captain of the 10th Division (and after he had retired, as well.)

Wazawai & Shukusei

In ancient times, the Yōkai Wazawai was "born" out of people's Fear of sickness and disease, thus serving as the original Yakubyō Gami and progenitor of all other Yakubyō Gami Yōkai. As people's Fear grew, thus strengthening wazawai, so too did their desire for protection against him grow; while this wish took many regional forms (Jinja Hime, Hōnengame, Amabie, etc.,) the creation of what would eventually become Shukusei was unique in its desire for an all-encompassing protection. This desire left Shukusei's original state as formless, voiceless, and nameless for quite a while; they were never seen nor heard, yet always managed to appear in time to stop Wazawai.
Although Wazawai grew infuriated by this new protector Yōkai, he also learned how to communicate with it after discovering it could understand him during an early confrontation. While the formless Yōkai seemed consistently capable of stopping his curses, Wazawai learned that the one day a year he was stronger than it was during the Hyakki Yagyō, using it to his advantage to overpower them and spread disease on a larger scale.

Over time, despite trying to find ways to kill this formless Yōkai, Wazawai would also speak with them often; after off-handedly mentioning one time liking the colors blue and green, Wazawai noticed that the once formless Yōkai now appeared as a slight shimmer of light blue-green in the air, easily mistakeable as a trick of the light. Because of this, Wazawai took to calling the Yōkai Shukusei (淑青, Shukusei, lit. Pure Blue-Green).

Shukusei slowly began gaining more of a form over the years; as Humans began ascribing traits of the twelve signs of the zodiac to them in order to explain their ability to ward off disease, Shukusei would adopt aspects of those animals into their corporeal form. The more Shukusei physically changed, the more Wazawai grew to despise them again, hating how they changed themself for Humans, but also how that meant (in his mind) that they were truly nothing of substance. At their core, they were a creature driven by the desires of those around them, thus incapable of their own desires.

Now with a form and a voice, Shukusei began to communicate with high-spec Humans capable of seeing them, leading to wide-spread tales about their grace and intelligence; Humans learned that even carrying an image of Shukusei could protect them from illness, and the knowledge spread from nobility to lower class. After telling Humans that their true name was Shukusei, Humans begin to write it with different kanji (宿星, Shukusei, lit. Star of Fate, Zodiac) after their chimeric appearance of all 12 animals of the zodiac.

This change in the spelling of their name culminated in a particularly violent Hyakki Yagyō where Wazawai, after hearing how Humans changed the kanji and meaning of their name, the name he gave them, nearly killed Shukusei, choosing to leave them for dead; Shukusei eventually healed and returned to full strength, as well as their own conviction to kill Wazawai in order to protect Humans for good.

Shukusei's Death

Over the course of the Edo period, Shukusei's life saw rapid growth and decay; while they finally had a form, they were relied and called upon by so many Humans that they began running themself ragged trying to keep all who invoked their image safe. Even as Human Belief in them remained high, their own Soul could not sustain the way they were travelling across the country constantly in order to stop not just Wazawai now, but any Yakubyō Gami who might threaten a Human wishing for their protection. Wazawai, as the only one who had consistently interacted with Shukusei over the last millennium or so, was also the only one to notice their severely deteriorating health.

As medical advancements grew over the period, Belief in Shukusei began to wane, until finally, as Wazawai went to purposely curse a household that historically used an image of Shukusei to protect them, he was greeted by neither image nor Shukusei themself. After cursing the household to less effectiveness than usual, he tried again at another household, only to once again find neither image nor actual Shukusei. Despite his best attempts to find them, it had seemed Shukusei had finally succumbed to their ailing health and died, with no one noticing except Wazawai.

The Last Hyakki Yagyō

Since before the signing of the Treaty, the Nurarihyon Chōshin would lead the Yōkai of the Human World in the Hyakki Yagyō every year. Although attendance remained high in the years following the Treaty, there was an undeniable tension ever-present; the threat of getting wiped out by Shinigami loomed over the heads of those participating, as well as knowing Umiko would act accordingly to punish any Yōkai who might step out of line during the affair. By the time of the Edo period, not only had growing concern over Shinigami retaliation put a damper on the parade's festivities, but the growing lack of Belief in Yōkai by Humans was drastically weakening the strength of all Yōkai.Roughly 200 years before the series start, Chōshin attempted to gather enough Yōkai to hold the Hyakki Yagyō as usual. However, nowhere near enough Yōkai responded to his Summons due to both his own growing weakness but also because many Yōkai had already left the Human World to return to Ashuradō. Wazawai had answered the Summons both as Chōshin's 1st Commander, and because he hoped to at least see Shukusei for it, that they might show up despite the odds of Wazawai's normally increased strength leading to their defeat and near-death each time- Instead, Shukusei remained gone, and Chōshin, having failed to gather enough Yōkai to join him, vanished, cursing Umiko's name and abandoning his Commanders and their Shōgun.

The Aftermath

Following the failure of the Last Hyakki Yagyō, the Yōkai of the Human World scattered across the winds; many returned to Ashuradō, while others still went into hiding. Some Yōkai who were capable of turning into Humans or otherwise blending in or hiding in plain sight chose to do that instead. Wazawai himself went into hiding over the next 200 years, all but haunting the country as he traveled around it, still searching for Shukusei, refusing to believe they had truly died.

The Jinja Hime Incident

Near the end of the Shōwa era (1926-1989) in 1980, 21 years before the start of the series, Humans began dying in Naruki City and Karakura Town under mysterious circumstances at an alarming rate, thus grabbing the attention of Gotei 13. On the cusp of a promotion to the rank of Captain, then-Lieutenant Isshin Shiba was assigned to the mission of uncovering the truth and stopping the deaths by his Captain and father, Issei Shiba.

Making his way to Naruki City to begin his investigation, Isshin discovered a Human woman who seemed to be having troubling bouts of amnesia- This woman was a young Reiko Kan'onji. Both wanting to help her and unable to shake the feeling that she might be connected to the case, despite her inability to see him without a Gigai, he ended up finding her again to work together and help each other.

As they investigated, Isshin and Reiko not only uncovered the truth behind the mysterious deaths, but also the truth behind her amnesia- And learned that the truth was one and the same.

Years ago, while Reiko was a high school student, she made the decision to leave her family's home in Hakone in order to attend Tanigawa High School, a private school in Naruki City, for her third year. At that time, Isshin and Reiko had actually met due to a rise in Hollow appearances shortly after she arrived; he had been assigned to investigate the increase in Hollows, only to discover that Reiko's innately high Reiryoku as a Quincy and Han'yō had drawn in a higher number of spirits, and thus more Hollows. In Hakone, Reiko never experienced this due to the protective seals her parents had placed across the ryokan they ran; upon moving to Naruki City, Reiko wanted to fit in, choosing to not use the ofuda her mother had given her and leading to the predicament Isshin found her in.

Isshin managed to clean things up and convinced Reiko to use the protection tags before returning to Soul Society, but the two stayed friends, and he visited her often whenever he got a chance to go on a mission to the Human World. Years later, after Reiko graduated, Isshin ended up taking an extended mission in order to spend time with her, which resulted in his father, Issei, showing up to drag him back to Soul Society himself. Upon seeing Reiko, Issei immediately recognized her as a descendant of Umiko, and chose to sever Reiko's spiritual connection and wipe her memory of Isshin entirely before leaving with Isshin. Upon returning to Soul Society, Isshin chose to wipe his own memory of her, blaming himself for everything that had happened to her. However, Issei only managed to seal Reiko's Quincy abilities, which allowed her full Yōkai abilities enough room to awaken.

With the traumatic loss of Reiko's powers and memories, it triggered a Seishin Hōkai in her- The nature of Jinja Hime is that they can predict times of famine or misfortune, but seeing a Jinja Hime, either in person or a picture of one, can help protect you- When an 'attack' on someone would happen, Reiko was essentially possessed by her Yōkai ancestry, lashing out at people who had never seen her face before, only to become lucid much later, having no memory of what occurred or how she got to where she was.

Now knowing the truth, Isshin helped calm Reiko and stop the Seishin Hōkai, preventing more deaths; when he returned to Soul Society, he was vague in his report, not mentioning Reiko by name, but simply referring to a Jinja Hime, which led to the incident's eventual common name.

The Hyakki Yagyō Returns

Following the end of the Winter War, after the loss of Ichigo Kurosaki's powers, the number of Yōkai present in the Human World very slowly began to increase. Reports of seeing Yōkai in passing began appearing in Shinigami mission reports, but due to the nature of the Treaty, Captain-Commander Yamamoto made no motion to act- Especially because the Yōkai had not done anything other than start to appear again. With the slow rise in Yōkai numbers, Reiko Kurosaki formed a group out of a small number of Yōkai she'd befriended in Naruki City, operating out of ramen shop Takonoko.

Mayuri's Research

While Mayuri Kurotsuchi has always had a vested interest in Yōkai, their research took a particularly interesting turn following the arrival of Kōsuke Kurosaki in Soul Society alongside Ichigo Kurosaki and the other Karakura Town kids. Although at the time, Kōsuke himself was still unaware, Mayuri learned after analyzing him during the "rest period" following Sōsuke Aizen's defection (and before the kids returned to the Human World,) that Kōsuke was a Han'yō. Utilizing bacteria they had placed upon him as they had done with all the others, he was able to continually view and analyze information not just about Kōsuke's abilities as a Yōkai, but also information about those around him- Namely his mother Reiko Kurosaki and older brother Isshō Kurosaki.

During the final fight against Sōsuke during the Winter War, Kōsuke entered into a forced Seishin Hōkai following Isshin Kurosaki's instructions for him while in the Dangai with him and Ichigo; utilizing the information they gained from the Seishin Hōkai, Mayuri began to learn more about this ability's nature, as it was something with extremely limited mentions in the Daireishokairō, much less the records of nobility that they were able to gain through the use of microscopic surveillance insects infiltrating where such records were kept. However, even with all this at his disposal, Mayuri knew their information was still woefully limited in its scope.

During the 17 months following the loss of Ichigo's powers, Mayuri began to do some more "hands-on" research; unable to fully study Kōsuke (or his family), Mayuri tasked Torue, Head of the S.R.D.I. Capture Department, with leading a small task force into the Human World to attempt to capture a Yōkai specimen for Mayuri to take apart. It took some time, but eventually they were able to successfully find a specimen to bring back to Soul Society alive.

Although the original Treaty from 1,000+ years ago clearly stated that Soul Society and Yōkai could not interfere with each others' affairs, and Mayuri capturing and dissecting Yōkai certainly qualified as breaking the Treaty, it is of his belief that the Treaty was already broken from the very moment Kōsuke set foot in Soul Society during the rescue of Rukia Kuchiki.

Over the remaining 17 months, Mayuri has been hard at work studying Yōkai of a multitude of varieties, trying to glean what information he can from them; the one thing that they have come to a standstill on is the one physiological element which remains the same in every Yōkai, regardless of species or build, organic or inorganic in genetic makeup: A very small, unidentifiable gland located just below the cerebellum (or varying equivalent of it), attached to it in an almost parasitic manner, causing them to call it the Kisei-sen in the meantime.

The Hō'ō's Blessing

Meanwhile, in Karakura Town, Reiko finds and rescues an injured Hō'ō; in return for her kindness, the Hō'ō offers to give Reiko a blessing, but Reiko only wishes for her son Ichigo to find peace, knowing he has been silently struggling with depression after the loss of his powers and connection to Rukia and Soul Society. Because of Reiko's wish, the Hō'ō gives Ichigo a small ember of power, sensing his desire to help and do good again- However, Ichigo and Reiko are not initially made aware that this is how the Hō'ō chose to interpret Reiko's wish. Before the Hō'ō departs, though, it gives Reiko a word of warning, mentioning an ill wind on the rise, before flying off and vanishing completely.

Reiko's Vision

That night, after rescuing the Hō'ō, Reiko's sleep is disturbed by a haunting vision of the Hyakki Yagyō, only instead of it appearing as it had in ancient paintings of the event, it is taking place within a modern Karakura Town- Finally settling on the illuminated silhouette of Chōshin through the curtains of his palanquin. Carefully, Isshin wakes Reiko up from the nightmarish premonition, before she lets him know that she believes Chōshin is seeking to bring about the Hyakki Yagyō once more.

Chōshin's Summons

Shortly after Reiko's Vision, Chōshin makes his move, sending out his Summons to all the Yōkai and Oni not only in the Human World, but even managing to reach those present in Soul Society and Las Noches. His Summons, although sounding like an invitation to partake in the Hyakki Yagyō, are actually a supernatural compulsion, forcing all who hear him to obey and join the parade. At least, that would be how it worked if not for the presence of Reiko; her status as a Jinja Hime offers a degree of protection to those who know her face or possess a picture of her, preventing those such as the Yōkai of Takonoko or her own family from hearing Chōshin's Summons.

Others, unfortunately, are not so lucky- Uno, the Yōkai of Las Noches and bondmate of Coyote Starrk, hears the Summons and is unable to fight the compulsion it brings; concerned for them (but also for their infant son, Arrancar Han'yō Lope,) Coyote reaches out to Kisuke Urahara in order to help break Uno from the Summons' command. Together, along with Lilynette, the family travels from Hueco Mundo to the Human World, staying in Urahara Shop while Kisuke and Sorano work together to find out what is going on. In Soul Society, while working alongside Mayuri, Akon also hears the Summons, getting permission to head to the Human World and Urahara Shop with Monsai Shikakubo in order to help Kisuke find out more.

Yamamoto's Orders

With the ever-growing concern regarding the appearance of Yōkai, and Mayuri finally reporting their findings to Yamamoto, the Captain-Commander sends out his orders to the officers of Gotei 13: Some get assigned to research Yōkai within the Seiretei's own records, such as Sasameki Kyōraku and Jūshirō Ukitake, while others get assigned to the Human World to get on-the-scene information, such as Shunsui Kyōraku. All of the Seireitei is ordered into an all hands on deck state, with everyone expected to help in some capacity. While all this is happening, Rukia has been working hard trying to figure out how to help Ichigo regain his Shinigami powers.

The Race Against Time

The Summons now released, Humans, Yōkai, Arrancar, Quincy, and Shinigami alike work together in order to prepare before the Hyakki Yagyō can fully begin. As everyone scrambles to be ready, Chōshin's Summons has reached his former Commanders and their Shōgun, leading them all to make the long trek towards Karakura Town. There isn't much time before the Hyakki Yagyō is set to start, leaving everyone in a race against time.

Fights

Han'yō

Han'yō (半妖, Han'yō) are demi-Yōkai. It is a term used to refer to any being who possesses Yōkai or Oni genetics, yet whose origins do not come solely from Yōkai creation or ancestry. Humans, Quincy, Souls, and Hollows are all capable of possessing Yōkai ancestry, and thus being labeled as Han'yō.

Yōkai Powers

Yōkai powers are as vast in nature as Yōkai themselves. Their strength is drawn from two main concepts: Belief and Fear. Prior to the final Hyakki Yagyō, Belief in Yōkai was at an all-time high, lending to their powers being much stronger, regardless of if people could see them. So long as people believed in the existence of Yōkai, Yōkai were able to feed off that strength to empower their abilities.

More than Belief, however, it is discovered over the course of the Arc that Fear serves as a far more lucrative source of power- While Belief is specific, requiring Humans to believe in the existence of Yōkai, Fear is far more broad in its scope, allowing them to draw strength from widespread Fear in general.

Yōkai powers can be split into two groups; abilities they can use regularly, and abilities that they need to enter a special state for, either group predicated on and specific to their Yōkai species.

Because Yōkai are (primarily) born either directly from nature or in Ashuradō, when they die, they are either reborn into nature or their Soul returns to Ashuradō- However, in some rare cases, Yōkai can be reborn as a Human.

Seishin Hōkai

Seishin Hōkai (精神崩壊, Spirit Break), is the first stage of an ability unique to Yōkai. When a Yōkai experiences powerful enough feelings of stress, pain, or distress, those feelings can trigger a Seishin Hōkai, allowing the Yōkai to transcend their limits and gain immense power at the cost of their own emotional and mental states. In modern times, Yōkai are far weaker than they were in the past; prior to the end of the Edo period, Yōkai drew their power from the Belief that Humans had in them. As Humans stopped believing in Yōkai, so too did Yōkai powers wane. However, an activated Seishin Hōkai has the ability to supersede the need for Belief.

Kanzen'na Jiga no Shi

Kanzen'na Jiga no Shi (完全な自我の死, Complete Ego Death) is the second stage of a Seishin Hōkai. This ability is a means for Yōkai to sacrifice their sense of self in exchange for powers which far exceed the limits of a Seishin Hōkai. It serves as the true final resort of Yōkai, as the activation of this stage generally results in the Yōkai's death.

Hyakki Yagyō

Hyakki Yagyō (百鬼夜行, Night Parade of One Hundred Demons) is the name of the yearly procession of Yōkai and Oni as lead by Supreme Leader Chōshin. Despite being called a parade, the event is more akin to a riot, with its participants causing chaos in the streets; during a Hyakki Yagyō, Humans would warn each other to stay indoors and not look out their windows, lest they be dragged into the streets and tormented or killed. Since its inception over a millennia ago, every year Chōshin has held the Hyakki Yagyō in a different village, town, or city of Japan, with his intention being to spread chaos as far as possible across the nation.

Supreme Leader

Supreme Leader (怪物の親玉, Kaibutsu no Oyadama, lit. Supreme Commander of Monsters) is the title of the Nurarihyon Yōkai, Chōshin. As Supreme Leader, Chōshin reigns over the Yōkai of the Human World, Hueco Mundo, and Soul Society- Every realm except Ashuradō, which is overseen by the Dragon Lord Ryūjin. Both due to his role as Supreme Leader and his Nurarihyon abilities, Chōshin is able to telepathically communicate- And, in essence, command- The Yōkai he holds dominion over through his Summons.

Summons

The Summons (召喚, Shōkan) is an aspect of an ability possessed by Chōshin which allows him to not only communicate en masse with all Yōkai and Oni not in Ashuradō, but can specifically grant him the ability to control them. While using his ability called Master of the House (一家の主, Ikka no Aruji), Chōshin is able to command Yōkai and Oni telepathically by using this ability to "invite" them to partake in the Hyakki Yagyō. This "invite", known more commonly as the Summons, compels all who hear it to heed his call, with very few exceptions which grant a degree of protection against the compulsion. Those protected will still hear the Summons, but will not feel the compulsion or need to obey.

Sōshinki-sen

The Sōshinki-sen (送信機腺, Sōshinki-sen, lit. Transmitter Gland) is a very small, biological gland located just below the cerebellum (or varying equivalent of it) within Yōkai and Oni. Human-like, flora-like, fauna-like, former inanimate objects- Yōkai regardless of origin all possess the same gland. It was discovered by Captain Mayuri Kurotsuchi while capturing and dissecting Yōkai leading up to the Arc; initially, he believed the gland to be part of the cerebellum, but later learned it was more akin to a parasitic attachment, leading to them calling it Kisei-sen (寄生腺, Kisei-sen, lit. Parasitic Gland). As their research revealed the true nature of the gland's functions, however, he renamed it to Sōshinki-sen.

It is not until the start of the Arc, when Chōshin sends out his Summons to all Yōkai and Oni not located in Ashuradō, that Mayuri finally begins to understand just what the Sōshinki-sen does: It serves as a means for Yōkai "higher powers" to mass communicate with Yōkai, similar to both the Tenteikūra Kidō spell, as well as the Shikai ability of Sasameki Kyōraku's Kinshitai.

Mayuri then begins exploring the depths of this connection, as well as how to hijack it for himself, eventually using it to connect directly to Akon (much to Akon's horror) while he is in the Human World with Monsai Shikakubo, working with Kisuke Urahara at Urahara Shop.

Commander

Commander (将星, Shōsei) is a rank and title given to the six strongest officers in Chōshin's army. Commanders are incredibly powerful and dangerous, on par with Captain-level Shinigami in strength.

Shōgun

Shōgun (将軍, Shōgun), is a rank and title given to officers hand-picked by Chōshin's Commanders. Each Commander is allowed to pick two Shōgun to have under their authority, with the first ranked higher than the second.

Onmyōji

The concept of Onmyōji is an ancient one, and while it was an official governmental office, within the world of Bleach "true" Onmyōji were particularly rare; those within the government who held the title of Onmyōji still existed, but those who possessed genuine magical ability gained them through a special pact.

Unlike the much larger grouping of Quincy, "true" Onmyōji were extremely small in number and didn't form any sort of cohesive organization, simply choosing to operate under the larger umbrella title of "governmental" Onmyōji. They gained their abilities to combat Yōkai by entering into a pact with one; these Yōkai could vary in origin, or intent- Some were ones who simply wished to give Humans a means of protection against more malicious Yōkai, while others still were more akin to tricksters, wanting to sow even more seeds of chaos into the natural order of things. This pact, however, was a double-edged sword; while it gave Humans the ability to combat Yōkai through tools or use (or subjugation in some cases) of Yōkai, the spirit who initiated the pact in the first place could just as easily (and often violently) take it away if it felt disrespected or mistreated by the Onymyōji. Onmyōji and their Yōkai formed a symbiotic relationship, with "ideal" pacts involving a mutual trust and respect.

As time went on, and political machinations (and Western influence) began to take root, Onmyōji lost their governmental standing in 1870; however, most "true" Onmyōji had already lost their abilities due to the overarching lack of Belief in Yōkai near the end of the Edo period. With the general populace losing Belief in Yōkai, Yōkai strength waned, and in turn the strength of Onmyōji waned as well. Even before this, though, many Onmyōji had already lost their pacts with Yōkai due to mistreatment (either true or perceived), which lead to a small event that was seen as a targeted attack, where many Yōkai attacked or even murdered their Onmyōji in order to break the pact.

In the modern era of Bleach, Onmyōji in the truly mystical sense have faded into myth, their pacts with Yōkai long-forgotten. There are perhaps some who exist with ancient familial ties to Onmyōji, but this is, just as the Onmyōji were in the past, a rare occurrence.