turn back the pendulum
The events at the soukyoku mark Aizen's betrayal of Soul Society, but his words indicate that this deception had been going on for quite some time.
Chapters -100 through -97 "turn back the pendulum," so to speak, and give us an idea of just how deep Aizen's treachery runs. Taking place around 100 years
before the main storyline, these chapters tell the story of the promotion of Urahara to Captain of the Twelfth Squad and the ultimate creation of the beings
called the "Vizards." During this time, Aizen was serving as Vice Captain of the Fifth Division to Captain Hirako Shinji. As Vice Captain,
Aizen is a contrast to his rather laid back yet volatile Captain (prone to fighting with Twelfth Division Vice Captain Sarugaki Hiyori, who liked to kick Shinji in the
face even back then), giving him the outward appearance of a calm and respectful young Shinigami. Aizen seems to take an interest in the new Captain of the Twelfth, even sneaking around
behind Shinji as he has a late night talk with Urahara. Shinji quickly discovers Aizen eavesdropping, indicating that he sees something suspicious in his Vice Captain.
Urahara, we see, is establishing the Twelfth Division's Technological Development Institute. We also see Aizen's first meeting with
Gin, a child "genius" who is admitted into the Fifth Division at a very young age.
Ten years after Urahara becomes Captain, news spreads of some mysterious deaths occurring in Rukongai. Shinigami's bodies are disappearing, but their clothes
are left behind, fully intact. Ninth Division Captain Muguruma Kensei and Vice Captain Kuna Mashiro set up camp near the site of some attacks and are attacked
by a mysterious enemy. Other Captains and Soul Society elite are eventually sent to investigate the matter (including Shinji, Hiyori, Third Division Captain “Rose”,
Seventh Division Captain Aikawa Love, Kidou Corps Vice Captain Ushouda Hachigen, and Eighth Division Vice Captain Yadomaru Lisa). They are confronted by Kensei
and Mashiro, who have somehow turned into grotesque Shinigami-Hollow hybrids. After subduing the monsters, Hiyori suddenly begins to transform as well. But
before she can be stopped, a masked member of the Ninth Division cuts down the rest of the Shinigami and reveals himself as none other than Tousen. Shinji asks
how he can be disloyal to his Captain, just as Aizen and Gin enter the scene. Tousen is loyal, Aizen explains, "but that loyalty is strictly reserved for my orders."
Shinji scoffs and admits that he had been suspicious of Aizen for a long time and had chosen him as Vice Captain to keep an eye on him. He knew Aizen was behind
the mysterious attacks; it becomes clear that Aizen had been trying to create the Hougyoku years Urahara had, though Urahara became the one to perfect it.
But Aizen knew Shinji was suspicious, and used this suspicion to his advantage. He reveals that for the past month, the man Shinji thought was his Vice Captain was
an impostor: Aizen used his zanpakutou's complete hypnosis to make it appear as if another man was himself. He preyed on Shinji's distrust, explaining that had his
Captain treated him normally he might have actually seen Aizen's true intentions. The lack of trust between them created distance; this distance allowed for Aizen
to go about his business uninterrupted and prevented Shinji from ever truly understanding his Vice Captain, therefore never noticing the impostor. Given Aizen's
manipulative nature, it's likely that he played up his suspicious acts; for example, he probably allowed himself to be found eavesdropping on Shinji and Urahara.
Furthermore, Shinji didn't choose Aizen as his Vice Captain, Aizen explains. Aizen choose Shinji. Shinji and the others then begin to transform into Hollow hybrids.
Aizen prepares to kill off his test subjects, but Urahara and Kidou Corps Captain Tsukabishi Tessai appear and stop him. Aizen tries to explain that he came to help
his Captain, but Urahara recognizes the Hollow transformation. Aizen seems pleased, saying Urahara is "just the man I thought you to be," and he, Gin, and Tousen walk away. Tessai suddenly launches an upper level
kidou spell, but Aizen calmly blocks it with one of his own, to Tessai's surprise. He escapes.
Urahara and Tessai scramble to bring the transformed Shingami back to
the Twelfth Division headquarters, where Urahara reveals the Hougyoku, which will be used to stabilize the Hollowfied Shinigami. But Aizen and Tessai are soon arrested and brought before Central 46, accused of
experimenting with Hollow transformation. Urahara protests and blames Aizen, but the committee explains that Aizen never left Sereitei that night, and they have
overwhelming testimony to prove it. The verdict is thus: Tessai is set to be imprisoned, Urahara stripped of his spiritual power, and the transformed Shinigami to be
killed as Hollows. But Shihouin Yoruichi, then Captain of the Second Division, appears and rescues Tessai, Urahara, and the eight Shinigami. Urahara
then plans for them to escape to the normal world, where they will work on a plan to take down Aizen.