Magatsu Wahrheit: Hitotsu kiri no Madousho

The Gilbert family is said to have play a decisive role in the battle against the evil sorcerers during the Great Fire of the past. Ewald respected his uncle, who had become the general of the Holy Knights in this battle and had received a treasure from the royal family.
Ewald thought that the treasure, which was carefully kept in the treasure room of Gilbert's family, might be the "One and Only Grimoire" that trapped the evil power of the sorcerer. But then he learns that a book similar to this grimoire is being sold on the back market...?

(Source: Ichijinsha, translated)