The mission is to gather seven witches.

The Kingdom of Amelia, which monopolizes sorcerers and expands its territory with supernatural power, poses a threat to the small nation of Campusfellow. The lord of Campusfellow, Bud, takes unprecedented action. His plan is to gather the notorious witches scattered across the continent to counter Amelia. At the same time, news arrives that the Mirror Witch has been captured in the neighboring country of Lowe. To take custody of the witch, Bud sets off for Lowe with his attendants. Among them is Rollo, a boy raised as an assassin, known by the nickname "Black Dog." Thus begins a grand and ruthless dark fantasy like no one has ever seen.

(Source: Shogakukan)