Majo no Konin - Nise Hanayome to Reikoku Kishi no Hatsukoi

Neri is a witch living alone in the depths of the Blue Forest. She lives quietly, carrying with her sad memories of her mother being taken away as an "evil witch" when she was a child. One day, a young lady visits her, hoping to escape a political marriage. Neri is unsure whether to accede to the request, but as she listens to her story, her heart freezes. The young lady's marriage partner is Erwin, the man who had once abducted her mother. In order to find her mother's whereabouts, Neri uses the only magic she has to transform into a young lady, and decides to marry Erwin as a "fake bride." And so, married life between the ruthless Erwin and the masked witch was supposed to begin, but...

(Source: Comic Walker, translated)