Ultimate Mission: Reunite My Family

Luna Miller wants her husband and her child back. Both were killed by an enemy state, and she is a weapons expert with no way to undo it, until a system binds itself to her and sends her into the past, into the body of a noble household's legitimate daughter in a dynasty long gone. The terms are fixed: marry the third prince, put him on the throne, and only then may she go home and save the two of them. She does what the system asks. She builds firearms with her own hands, wins credit for the prince on one battlefield after another, and stays beside him through the fight over the succession. The prince, meanwhile, prefers her half-sister, born to one of the household's concubines. He demotes Luna from wife to concubine and makes her kneel to the girl. On the day of his coronation Luna is framed, taken prisoner, and paraded through the streets toward her execution. The prince hears what has happened partway through the ceremony.