Chapters 3-12 of the light novel detail the introduction of Sakura, a seemingly ordinary noble daughter, and her unexpected entanglement with the powerful Prince Kai. The narrative begins with Sakura enjoying a peaceful walk in the woods near her family's estate, the Audle house, while her family busily prepares for the coming-of-age ceremony of her younger sister, Aoi. Aoi is renowned for her beauty and is expected to attract numerous suitors, including a "noble personage."
During her walk, Sakura discovers a wounded, pure white magical beast. While tending to it, she overhears a conversation between two cloaked men on horseback. They discuss a failed mission and the death of someone named Sara, and one of them mentions the beauty of the Audle daughters, specifically Aoi, as a potential wife. Sakura, hidden with the white beast, realizes the man's high status and the coldness with which he discusses the death of a comrade. The white beast, sensing danger, reacts to the men, and Sakura, protecting it, is nearly discovered. After the men leave, Sakura offers the beast her blood to heal it, and it accepts. She advises it to flee before the upcoming celebrations, as hunters might be present. Sakura reflects on her own overlooked status as the second daughter, overshadowed by her beautiful elder and younger sisters.
The chapter arc culminates with Aoi's coming-of-age ceremony. Sakura, feeling out of place amidst the grandeur and attention focused on her sisters, retreats to a balcony. There, the white beast reappears and comforts her. Suddenly, chaos erupts as a magical beast is reported to have entered the hall. In the ensuing panic, Sakura instinctively shields the white beast from a drawn sword wielded by a man. She is impaled by the sword, but miraculously, no wound or blood appears. Instead, she is enveloped in a gust of wind and disappears. The man, Kai, claims her as his own, stating she is "his."
Sakura awakens in an unfamiliar room, finding herself in bed with Kai, a man with striking heterochromatic eyes (gold and black). He reveals he is Kai Rilunes, the second prince of Killingseek, known as the "Black God of War." He explains that the sword that pierced her chose her as its sheath, and to keep her by his side, he has declared her his wife and given her the Rilunes name. Sakura, overwhelmed and confused, learns that Kai is a powerful figure who, along with his elder brother, saved the kingdom. She struggles to accept her new reality as Kai's nominal wife, a role she feels ill-suited for, especially given her perceived ordinariness compared to her sisters.
Over time, Sakura begins to adapt to her new life, though she remains hesitant to fully embrace her position. Kai, while distant, shows subtle kindness, such as arranging for her childhood friend, Hotaru, to become her attendant. Sakura also learns that Kai's interest in her stems from her connection to the "Demon-Slaying Sword," which has chosen her as its sheath. She is expected to remain by his side, providing a stable presence for the sword. Despite her initial fear and self-doubt, Sakura begins to find a sense of purpose and belonging, even as Kai remains an enigmatic and powerful figure. The narrative hints at the ongoing threat of magical beasts and Kai's role as a protector of the kingdom.