Yona of the Dawn manga volume 20 features story and art by Mizuho Kusanagi.
 
Yona and Riri wind up in the Nation of Sei after being betrayed by a trusted acquaintance! Forced to work as slaves, the two stubbornly refuse to be controlled, but their situation is dire. In order to save Yona and Riri, Hak and the Four Dragons split into two groups and sneak into enemy territory!
 
Author: Mizuho Kusanagi