- Died: 1865
Katsui had been imprisoned by Sasu Iori, chief councilor to Yoshiakira's father Sô Yoshiyori, but was freed when a group led a coup in 1862, assassinating Sasu and installing Yoshiakira as daimyô. This group then entered the domain into an alliance with Chôshû han, which crumbled soon afterwards as the shogunate turned against Chôshû. Much of the benefits Tsushima had secured from the shogunate, with Chôshû's help, also disappeared as Chôshû and the shogunate came to blows in 1864.
Katsui thus decided to lead a coup against the very group which had freed him two years earlier. In a course of action which somewhat mirrored that earlier coup, Katsui led a group of men to the Tsushima domain mansion in Kyoto, where he confronted Ôura Sakubei, chief Tsushima official in Kyoto and son of Ôura Norinosuke, one of the leaders of the rival faction. In the ensuing skirmish, Sakubei was mortally wounded, and Katsui fled to Tashiro, one of Tsushima's tobichi fiefs on mainland Kyushu. Gathering up forces, he then returned to Tsushima and snuck into the daimyô's palace, capturing Sô Yoshiakira and seizing dominance within the domain's politics.
In 1864/10, Katsui closed Ôura's Nisshinkan, a training school and center of sonnô jôi ideology, and over the next several months killed nearly one hundred relatives and supporters of Ôura, or forced them to commit suicide. Ôura Norinosuke himself died in prison as the result of a hunger strike, and lost one grandson to suicide and one to execution.
- Robert Hellyer, Defining Engagement, Harvard University Press (2009), 227-230.