Mashiro’s dreaming about Kensei

Around 100 years before the current storyline, people in Soul Society started to disappear, but more than simple death, there were no bodies found; only the victims' clothes remained. The situation caused a great uproar in the Gotei 13 as investigative groups were sent out and not heard from again. Due to this, the 9th Division's command structure took it upon themselves to investigate the situation: Kensei Muguruma, then-captain of the Ninth Division, Mashiro Kuna, his lieutenant at the time, and their subordinates (comprising the 3rd to 6th Seats). They were all mysteriously cut down by a then-unknown assailant that succeeded in taking them all by surprise using some unknown method. When their Reiatsu disappeared in a manner similar to the previous disappearances, the Gotei 13 became worried and sent out a new investigation team personally hand picked by Captain-Commander Yamamoto to see what had happened. Captains Shinji Hirako of the 5th Division, Love Aikawa of the 7th Division and Rōjūrō Ōtoribashi of the 3rd Division, along with the Kidō Corps' lieutenant Hachigen Ushōda and Kyōraku's lieutenant Lisa Yadōmaru, all set out to find the missing Shinigami. Urahara's lieutenant, Hiyori Sarugaki was already on her way to collect samples of the disappearance site per Urahara's request. When the group finally arrived they found Hiyori and then themselves under attack by a Hollowfied Kensei and Mashiro. Even while working as a team they were overpowered by the brutal attacks from the former fellow Shinigami. When Hachi arrived, he bound Mashiro and Kensei using Kidō. Hiyori, now being held by Shinji, begins to succumb to the Hollowfication process and attacks Shinji without warning. The others were then surrounded by a pitch black darkness that also rendered them unable to sense each other, where they were systematically cut down. After the darkness is lifted Shinji is the only one still conscious to see that it was all Kaname Tōsen, Kensei's 6th Seat, using his Bankai against them. Shinji tried to question Tōsen about the betrayal of his captain while he and the others were going through various stages of Hollowfication. Aizen, Shinji's lieutenant, then shows up with Gin Ichimaru, Shinji's 3rd Seat in tow, explaining that he was following his orders. Aizen begins to explain how he was behind it all and that he had been manipulating Shinji from the start using his Zanpakutō's special ability. He then gave Tōsen permission to get rid of Hiyori, whom he attacked. Shinji, who refused to go down without a fight, engaged in combat with Tōsen. He initially gained the upper hand due to the increase in his power, but eventually succumbed more to the Hollowfication process. Aizen prepared to attack him, but was stopped by the timely arrival of the cloaked 12th Division Captain Kisuke Urahara and Kidō Captain Tessai Tsukabishi.