Haihi wa Futatabi Gyokuza ni Noboru: Akarutei Koukyuuishi

Miou Ryuu had been sent to lead a life of comfort as part of the Ryuu Family - maternal relatives to the imperial family of the Kingdom of You. Upon turning 18, it was decided that she would marry Setsuhou Shiba, the emperor of the kingdom. Miou had always been deeply in love with Setsuhou, for he had been her first love. Thus, Miou waited in the bedroom so they could spend their first night together as emperor and empress... But after waiting for what seemed like forever, Setsuhou arrives and hands her the freshly severed head of her aunt, saying that she had been an atrocious, inhuman, and evil woman! He then declares, "You are being dethroned. Thereafter, you shall be sentenced to death." It turns out that on the night of her wedding, the government had been overthrown in a coup, and Miou had been convicted of a crime and sentenced to execution for being a "sworn enemy of the entire nation"! Ten years have passed since then, during which Miou has been living imprisoned in the dungeon of the royal palace... though she still looks to be 16. That is when a certain young man arrives who completely changes Miou's life... Source: Shogakukan