Sometime in 2013, members of the Onmoraki-Gumi attacked a cell of Assassins located in Osaka, killing several of them, including their Mentor, Kenichi Mochizuki. Kenichi's wife, Saeko Mochizuki, took over the Osaka Brotherhood and retaliated against the Onmoraki-Gumi, wiping out their leaders and taking over the organization. Saeko and her Assassins then began using the Onmoraki-Gumi to blend into the city, believing that the Assassin Brotherhood and the yakuza had similar styles of working in the shadows and helping the common people.
On 4 December, the crew of the Assassin vessel Altaïr II arrived in Osaka, and discovered the Osaka cell's original hideout had been destroyed. They began investigating, suspected that the Onmoraki-Gumi were responsible, and began tracking them down. On 12 December, Emmanuel Barraza tracked a cash deposit to the Onmoraki-Gumi's new base, and Assassin leader Gavin Banks went with him there on 17 December to discover the truth. After an initial hostile misunderstanding with Kiyoshi Takakura, on 18 December Gavin and his cell met with Saeko, who explained to them what had happened.