Muteki no Ansatsu Madoushi, Last Boss Musume to Tabi o Suru @COMIC

Brad Lester, once renowned as the strongest mage, vanished from the public eye after the tragic death of his young daughter. While living a quiet life as a lowly day laborer, his beloved wife is murdered by the "World Peace Promotion Association," an organization seeking his power. Driven by a despairing rage after losing everything, Brad sets out for revenge—only to find Valburga, the leader of the Association, has been transformed into a little girl!? Furthermore, she has lost her memories and has nowhere to go. Haunted by the memories of his late daughter, Brad resolves to "help his arch-nemesis regain her memories." Visiting the Olford Ancient Magic Research Institute, the mouse-worshipping town of Brooke, and the blizzard-shrouded village of Lutz, their long journey across the land begins—. An adventure fantasy of the strongest mage and his "child" companion!!