Conspiracy's Masterstroke: Illusory Assassination is a major event of the Ryoka Invasion, revolving around 5th Division Captain Sōsuke Aizen faking his own death in order to distract the rest of the Gotei 13 while he plots to extract the Hōgyoku from Rukia Kuchiki.
After Rukia Kuchiki is located in the Human World, 5th Division Captain Sōsuke Aizen murders the Central 46 and uses Kanzen Saimin on the Central Underground Assembly Hall to make it appear as though the Central 46 were alive and in session so anyone who entered the chamber would not notice anything unusual. Aizen, 3rd Division Captain Gin Ichimaru, and 9th Division Captain Kaname Tōsen proceed to hide themselves in the Central Underground Assembly Hall and issue orders as though they were Central 46: sending 6th Division Captain Byakuya Kuchiki and Lieutenant Renji Abarai to ensure Rukia's capture, ordering Rukia's Gigai be immediately returned for disposal to separate her from the Humans, and deciding to execute her with the Sōkyoku in order to evaporate her Soul and extract the Hōgyoku within.
During this time, the three captains only leave the Central Underground Assembly Hall open for a few hours before and after the two captains' meetings; in the first captain's meeting, Aizen holds a cryptic conversation with Gin to arouse 10th Division Captain Tōshirō Hitsugaya's suspicion. Additionally, Aizen manipulates the Ryoka's infiltration of the Seireitei by posting surveillance in West Rukongai where they entered Soul Society, lowering the Sekkiseki wall around the Seireitei after their arrival, and sending Gin to drive Ichigo Kurosaki away personally, which leaves Kūkaku Shiba's Kakaku Taihō as their only means of entering the Seireitei and provides a flashy, engaging distraction for the Gotei 13 while Aizen begins his machinations.
In the evening after Renji's defeat at the hands of Ichigo, Aizen's lieutenant, Momo Hinamori, is informed by a Shinigami of the special wartime orders being issued by Captain-Commander Genryūsai Shigekuni Yamamoto in response that permit all captains and lieutenants to carry and use their Zanpakutō within the court, leading him to request that Hinamori be on her guard because the Ryoka could attack the barracks at any time after defeating Renji even though they have tightened their defenses. After the Shinigami departs, Hinamori remains kneeling in her room and wonders how the situation could have escalated to the point of them needing to carry their Zanpakutō before telling Aizen that she does not want to fight.
That night, as he writes a letter in his own room, Aizen notices Hinamori standing outside the sliding doors to his room and inquires if something is wrong. Apologizing as she enters Aizen's room, Hinamori requests a moment to speak with Aizen and acknowledges how rude it is of her to do so at such a late hour before bashfully promising to not fall asleep in front of Aizen, who merely covers her with his robe while questioning if she really thought he would cold-heartedly send her away for being rude and inviting Hinamori to come inside and stay until she calms down because she must have had a difficult day. Shortly afterward, Hinamori sits behind Aizen, and when the latter observes that he heard Renji is going to live, Hinamori expresses relief.
Aizen details how Byakuya was met with opposition when he called for Renji to be discharged and concludes that Renji will be able to rejoin the Gotei 13 once his wounds heal. When Hinamori asks him if he was part of this opposition, a smiling Aizen clarifies that no one in the Gotei 13 wants to see him discharged because he is a strong fighter and well-liked by everyone, leading Hinamori to mentally note that everything about Aizen soothes her fears as she expresses her happiness at having come here tonight and for being able to work under Aizen. Later that night, with Hinamori sleeping in his bed, Aizen steps outside, closes the doors behind him, and silently begins walking away.
The next morning, Hinamori bolts awake and apologizes for dozing off, only to find that Aizen is gone. When her alarm goes off, Hinamori expresses shock at the time and gets dressed before running out as she questions why Aizen did not wake her up if he woke up before she did and expresses uncertainty if she will make it to the assembly on time. Upon encountering a roadblock on the walkway, Hinamori jumps over it to take a shortcut and confidently declares that she will make it to assembly on time, but stops in her tracks and screams loudly upon seeing something ahead.
Split Under The Red Stalk
Alerted upon hearing this, 9th Division Lieutenant Shūhei Hisagi, 10th Division Lieutenant Rangiku Matsumoto, 3rd Division Lieutenant Izuru Kira, and 7th Division Lieutenant Tetsuzaemon Iba rush over to the East Holy Wall where they heard the scream emanate from as a concerned Izuru identifies it as Hinamori's voice. However, upon reaching a stunned and tearful Hinamori while inquiring what happened, the lieutenants are shocked to find her staring up at Kyōka Suigetsu, which Aizen has disguised as his bloody corpse pinned to the wall by his own Zanpakutō with the aid of Kanzen Saimin. Hinamori tearfully calls out to Aizen over and over while Iba, Rangiku, Hisagi, and Izuru stand behind her in stunned silence.
Suddenly, Gin inquires why they are making so much noise so early in the morning as he appears behind them. As she looks at Gin, Hinamori recalls Hitsugaya's warning for her to beware the Third Division, especially when Aizen goes out alone. A furious Hinamori exerts her Reiatsu and accuses Gin of having done this before leaping toward him and attempting to slash at Gin with her Zanpakutō, only for Izuru to intercept her at the last second by blocking her strike with his own Zanpakutō. When Hinamori questions why he has done this, Izuru asserts that he cannot allow anyone to draw a sword against his captain for any reason as the 3rd Division Lieutenant.
Though Hinamori tearfully begs him to move aside twice, Izuru staunchly refuses both times, prompting an enraged Hinamori to demand that he move while Izuru emphatically refuses once more. Releasing her Shikai, Tobiume, Hinamori unleashes a fireball that creates a large explosion, knocking a shocked Izuru back while she assumes a battle stance. Though Izuru criticizes her releasing her Zanpakutō here and asserts that this is no place to resolve personal matters, Hinamori does not respond and launches another fireball at him, which Izuru dodges before noting that he now has no choice but to treat Hinamori like an enemy.
However, as Izuru leaps into the air and releases his own Shikai, Wabisuke, while coming down onto Hinamori, their clash is interrupted by Hitsugaya, who blocks Izuru's subsequent strike and pins Tobiume to the floor with his foot as he tells both Izuru and Hinamori to not move. With Hinamori stunned by his arrival, Hitsugaya commands the other lieutenants to arrest her and Izuru, which they do, and declares that he will report this to Yamamoto before ordering the lieutenants to take them away. While Hinamori and Izuru are taken away, Gin apologizes to Hitsugaya for Izuru troubling him, but Hitsugaya observes that Gin just tried to kill Hinamori, and though Gin denies knowing what he is talking about, Hitsugaya promises to kill him if he makes Hinamori bleed, prompting Gin to claim that Hitsugaya is scaring him before noting that he should keep an eye on Hinamori so no misfortune befalls her.
Some time later, in the first Special Confinement Gaol within the Fifth Division barracks, Hinamori sits with bags under her eyes and looks up to see Rangiku approaching. As Hinamori wonders why she is here, Rangiku hands her a sealed letter and reveals that it was found in Aizen's room and is addressed to Hinamori herself, to the latter's surprise. With Hinamori recalling Aizen writing a letter while they were speaking last night, Rangiku remarks that it is a good thing Hitsugaya found it because anyone else would have submitted it as evidence and prevented Hinamori from receiving it. Admitting that she does not know what it says, Rangiku tells Hinamori she should be honored her captain's last words were for her and should read the letter with appreciation.
After tearfully thanking Rangiku for this as she departs, Hinamori unties the string binding the letter and opens it, where she finds Aizen stating that her reading this means that he was unable to return before apologizing for how much grief he has caused her, as he has no words to express his gratitude. Aizen's letter details how he never spoke to her about his concerns because he did not want her to get involved, and as he expresses hope that Hinamori will forgive him for involving her now, Aizen reveals all he has discovered so far in the letter to Hinamori, whom he trusts more than anyone else, because he is likely no longer alive, leaving Hinamori astonished and sweating in shock.
Having successfully faked his own death, Aizen retreats into the Seijōtōkyorin and uses the Daireishokairō to look through Kisuke Urahara's research on embedding foreign objects inside Souls, believing that he will need a backup option in the event that the Ryoka succeed in stopping Rukia's execution and thus prevent the Sōkyoku from evaporating her Soul. After discovering a method which will allow him to retrieve the Hōgyoku directly by tampering with the composition of Rukia's Soul, Aizen reveals himself and makes his move on the day of her execution.