The Duke of Ashleyan’s Contractual Marriage

After the ‘Tyrant King’ killed our parents, we managed to defeat him in a coup d’état. My younger sister became Empress and I became the Duchess who ruled the beast-infested Northern territory. We took our revenge and my younger sister has become the Empress, so there’s only one task left – the oath I made to God right before helping my younger sister. In order to fufill that oath, I’ll have to get married. But for someone like me who has never had a relationship before, ‘love’ is more difficult than war or fighting a dragon. However, the illegitimate son of the tyrant, the Prince, has just proposed a contractual marriage…?
“Three years. Please just stay married to me for three years. I will divorce you right after that.”
What is he saying? And yet, why does he keep catching my eyes?