Adachi Yasumori was the grandfather of the regent, Hôjô Sadatoki. Following the death of Hôjô Takayori, Yasumori, an ambitious and gifted admistrator, was named Osso Bugyô, or Appeals Magistrate. Yasumori had also been the chief of the Rewards Office, or Go'on Bugyô. In this capacity, he granted a hearing to Takezaki Suenaga, the creator of the Mongol Invasion Scroll.
Yasumori was a careful politician and maintained good relations with both the Hôjô regency and the Minamoto house. His influence within the warrior government in Kamakura was significant. Unsurprisingly, Yasumori gained a number of enemies, the most powerful of whom was Taira Yoritsuna, the Hôjô family's house steward. On the 11th month of 1285, Yoritsuna attacked the Adachi compound. The fighting lasted for some five hours but in the end Yasumori was forced to commit suicide. He was joined in this by all of his family and many of his supporters. Among them were men from the Banno, Osone, Ogasawara, Ueda, Kobayakawa, Mishina, Ashina, Futokorojima, Tsunashima, Ikegami, Namikata, and Nikaido families.
The so-called Shimotsuki affair spread to bring misfortune to even more clans and the final toll of suicide deaths in unknown.
The reasons for Yoritsuna's attack are hazy but possibly Yasumori's heir, Munekage, was plotting to take the post of Shogun (he claimed descent from Minamoto Yoritomo.) Just as likely, if not more so, is that Yoritsuna generated these charges as a pretext to do away with his hated rival Yasumori. Yet Yoritsuna himself would be done in by rivals in 1293.