THE STAND-IN BRIDE: MY FAKE MARRIAGE TO THE ICHIJO FAMILY

After her mother's passing, Mio is left completely and utterly alone. One day, a man shows up out of the blue, taking her in after claiming to be her father. Mio is soon used as a tool for a political marriage. She is told to marry into the Ichijo family, and to give birth to a son within a year. The Ichijos are prestigious; they have ties to the imperial family, work under the emperor, and have served as exorcists. However, the head of the Ichijos, Kazuomi, shows no interest in Mio, and even refuses to speak to her. A fate worse than death awaits Mio if she can't give birth to a son in time... Mio spends day to day in despair, but one day, she learns of a major secret about the Ichijo family. Kazuomi's tune begins to change once it becomes clear that Mio also possess a special power that connects her to this secret! (Source: MangaPlaza) — Description from AniList community (anilist.co).