D9 -Daemon Master of Lost Ark-

is a Japanese light novel series written by Yu Ueno, with illustrations by Kokonoka. The series began publication on January 10, 2014 by ASCII Media Works under their Dengeki Bunko publishing imprint.

Synopsis
Thousand years from the "end of the disaster" ── A continental faradice dominated by demons, magic, and a group of lost machines. The boy Soma signed a contract with the beautiful girl devil Merville and traveled the world together. Looking for a real brother who summoned the devil and destroyed his hometown.

During the journey, they stop at Derna, the city of the renowned demon scholar. A foreign maid and a demon scholar like my father. There, they are involved in a big incident that shakes the city. The devil-possessed boy Soma and the girl devil Mel. The two do not yet know that the journey will hold the "fate of the world."