The Untalented Lady and the Marquis's Infinite Devotion

In a world where everyone—young and old, male and female—possesses magic, Charlotte is the only person born without a drop of it. Deemed worthless, she fails to fetch even the lowest price at an auction and is on the verge of death. Suddenly, a nobleman named Arthur appears and offers to take her in for 30 million. Claiming to be her guardian, Arthur falls in love with Charlotte at first sight. He explains that her magicless body is the key to saving him from the agonizing "excess magic" that plagues him, and he proposes marriage. Though bewildered by Arthur’s sweet and doting nature, Charlotte hopes to finally find a place where she belongs. However, saving Arthur requires a "certain act"—?