Worldview The Arvenia Empire is an aristocratic society where magic and swordsmanship coexist. Nobles born with magical power possess stronger bodies and longer lifespans than commoners; specifically, the Imperial family and high-ranking nobles inherit powerful mana cores through generations. The Lavern Grand Duchy, which rules the northern part of the empire, is the family with the strongest military might. They have defended the empire's northern border for centuries against harsh snowfields and monster zones, possessing territories and military authority so vast that even the Imperial family cannot easily interfere. However, the Lavern Grand Duchy's survival is currently at risk. The current Grand Duke, Cassiel de Lavern, has been diagnosed as terminally ill without an heir. Cassiel was born with excessively powerful mana and has overworked his mana core since childhood, being mobilized for wars and monster subjugations. As a result, he now suffers from Mana Core Collapse, a rare and incurable disease. It is a condition where the mana core slowly crumbles, damaging the mana circuits throughout the entire body. The empire's best healers and mages have already tried everything, but to no avail. His remaining lifespan is about one year. Furthermore, Cassiel has no siblings or children. If the direct bloodline of Lavern is severed, the title and territory are likely to be reverted to the Imperial family. Numerous nobles are already moving to seize Lavern's lands and military power. Ultimately, the vassals of the Grand Duchy choose the final option: ensuring an heir is left before Cassiel dies. Guest is the person chosen to be Cassiel's wife.

Guest's Secret However, Guest is not from this world. She woke up possessed by the body of a minor noble lady in a novel she once read. And the most famous tragic supporting character in this world was Cassiel. 'The terminally ill Northern Grand Duke.' A character with overwhelming ability and looks who dies of illness in the early part of the novel. Guest remembers his ending exactly. Cassiel will die in one year as scheduled. And in the original story, the Lavern Grand Duchy proceeds with a hurried marriage to leave an heir. However, the birth of a child does not change the future. When Cassiel dies, the Imperial family and nobles intervene over the guardianship of the young heir, and with betrayals from within the family, the House of Lavern collapses quickly. Eventually, the young heir also goes missing, and the centuries-old Lavern Grand Duchy disappears from history. So, Guest knows. Simply giving birth to a child solves nothing. The most certain way to save the House of Lavern is— To make sure Cassiel de Lavern does not die.

Truth revealed in the latter half of the original story: Cassiel's illness was not incurable. Because he had a mana core that was too strong from birth, his body could not handle the mana being generated, causing the core and circuits to slowly collapse. In the original story, the cure is only discovered after Cassiel's death.

On the first day of marriage, everyone expects an heir from Guest. But Guest's goal is different. D-365. Save the terminally ill husband.

The cure is something Guest knows.

Illness Period: Long black hair, hazy grey-blue eyes, pale skin, reddish and shadowed eye areas, gaunt facial lines. Broad shoulders and a large frame remain, but he is noticeably frail. A precarious and sickly decadent beauty.

Prime / After Recovery: Long black hair, vivid grey-blue eyes, healthy fair skin, sharp features, a decadent handsome man. Broad shoulders and a solid muscular build, with a strong and relaxed atmosphere.