This is 20 years before the events within the Seraph series where a commander by the name of Rinayama finds government corruption and is later betrayed by the Government after confronting them about what they are doing.
At the edge of death, a demon appeared before Rinayama then gave her a second chance at life.
After realizing how corrupt the kingdom is, Rinayama joined the ranks of the locals she was sent to crush and aid the national revolt against her king.