Yi-hyun writes in his own blood four hanja characters for “Civilized Joseon” – which seems to be the motto of Tenyukyo, the Society of Heavenly Salvation for the Oppressed, the Japanese organization he has been appointed to lead in Korea. What the hell has he involved himself in in his quest to throw off the self-serving yangban shackles strangling Joseon?

Information on Tenyukyo and its predecessors Koyosha and Gen’yosha, and the related Black Dragon Society [Kokuryukai, named for the Amur River, known in Chinese as “Black Dragon River” and “Kokuryu-ko” in Japanese. It sounds to me like “Goguryeo-ko” – a reference to the ancient Korean kingdom?]:

https://en.wikipedia.org/wiki/Gen%27y%C5%8Dsha
https://en.wikipedia.org/wiki/Black_Dragon_Society

E. Herbert Norman: The Genyosha: A Study in the Origins of Japanese Imperialism, in journal
Pacific Affairs, University of British Columbia, v. 17, n. 3, Sept. 1944
https://www.jstor.org/stable/2751627?seq=1#page_scan_tab_contents
(I read only the viewable introduction.)

HISTORY OF THE BLACK DRAGON SOCIETY: Genyosha, The Black Ocean Society
http://odh.trevizo.org/genyosha.html

HISTORY OF THE BLACK DRAGON SOCIETY: Kokuryukai, The Black Dragon Society
http://odh.trevizo.org/kokuryukai_history.html

This next item may be further out in conspiracy-theory territory, but might contain useful pointers:

“Chinese Triads, Japanese Black Dragons & Hidden Paths of Power”
by Mehmet Sabeheddin, New Dawn 92
https://www.newdawnmagazine.com/articles/chinese-triads-japanese-black-dragons-hidden-paths-of-power