NIEHIME NO KONIN: MIGAWARI OUJO WA TEIKOKU DE SAIAI TO NARU

"This time, I'll protect them. That child's life, and my own happiness." Princess Lucia, raised in confinement as the daughter of the former king after a coup, is suddenly dragged out into the outside world and married off to the ruthless Emperor Ricardo, a man of terrifying looks and personality, in place of her cousin Marguerite. Experiencing a loveless pregnancy and childbirth solely for the purpose of producing an heir, Lucia feels happiness for the first time with her own child, but is murdered by an assassin sent by Marguerite, who wants to use Lucia's position and child. However, at the moment of her tragic death, time is miraculously rewound to when she was imprisoned. With a determination to avoid assassination and protect her precious child at all costs, Lucia marries Ricardo once again. Then, she begins to see an unexpected side of Ricardo, who she thought was cruel and devoid of love...!? A substitute princess who was married off as a sacrifice to the ruthless emperor and assassinated—a time-leap drama of reversals and a doting romance!! (Source: Echo, translated) — Description from AniList community (anilist.co).