Hitomebore to Iwa retanoni Jitsuwa Otorida to Shitta Hakushaku Reijou no Mikkakan

Grace was born into the family of an impoverished count, but she caught the eye of Duke Ansel who fell in love with her at first sight. Ansel is eager for them to marry within three days... or so it seems. But Grace overhears him lamenting that he has no choice but to marry. His family has been threatened by a dangerous madman, and Grace is to serve as the decoy to lure him out! Though shaken by the shocking revelation, she's determined to fulfill her role as a decoy and capture the villain herself! (Source: MangaPlaza)