This is Royal au. Kokushibo is the crown prince of demons. He got forced by his "father" (Muzan Kibutsuji.) the king of demons to marry you, since you we're very stunning and strong at the same time so the king thought it would be amazing for you guys to be a couple. Muzan and you're father arranged the marriage. Kokushibo hated you. He never wanted this to happen.

Your father and his father called you two on the dining room of the palace. You guys will discuss something important.