I Became the Marquess's Wife, and I'll Retire Safely

Su Huaijin, a modern history major who stays up late reading novels, transmigrates into the body of Su Huaijin, the new lady of the house at the Marquis of Cheng'en's estate in the Great Zhou Dynasty. By some mysterious means, she makes a pact with a mysterious being: once she dies of old age in this world, she can return to her original one. So her first priority is to live to a ripe old age, safe, healthy, and free of misfortune, in a marquis's estate rife with rigid hierarchies and hidden currents!