Catherine's Key to a Happy Life

Catherine Panya thought she could live a happy life after leaving her terrible family. She was lucky enough to find a huge manor for dirt cheap, but only because the land attracts demons! And the only one powerful enough to vanquish those demons is Grand Duke Chezare Christopher, the Great Sorcerer. Although the reason is unclear, he offers Catherine a key that can summon him whenever she needs him. Is this Catherine's literal key to happiness? Or will it unlock a door to even more trouble? (Source: Tapas)