I Don't Want to Get Married, so I Have No Choice but to Become a Duke

[Western Fantasy Master and Servant Revenge] My life suddenly changed one day. As the illegitimate son of a Celtic Duke, I was supposed to spend my life peacefully on a farm in the countryside, but I was brought back to the Duke's palace by the Duchess. My destiny was not to be a marriage tool to maintain family relations, but to participate in the Duchess's usurpation plan to become the real new Celtic Duke. The Duchess's child died unexpectedly in the Custer family's territory, but the Custer family's reason was incredible. The Celtic Duke was unwilling to go to war with it because of his identity as an ally. My goal is to replace the current Celtic Duke and launch a war of revenge against the Custer family until I complete the revenge of the Duchess... This is a path of no return and I have no choice but to become a Duke. The three major families chapter officially begins!