I Don't Want an Arranged Marriage, So I Have No Choice but to Become a Duke

[Western Fantasy Master-Servant Revenge Story] My life changed all at once. As the illegitimate child of Duke Celt, I should have lived out a peaceful life on a rural farm. Instead, the Duchess brought me to the ducal estate. My fate was not to become a marriage pawn used to maintain the family's relationships, but to become the true Duke of Celt.

The Duchess's children died unexpectedly in the territory of the Caster family, claimed by a demonic fever, while Duke Celt did nothing. So Crane Wade and I were ordered to investigate the cause of her children's deaths and take revenge on the Caster family, the ones behind it all. In return, she promised to hand me the ducal throne, asking only that I fulfill her wish. But as events unfold, the truth turns out to be far more complicated...