The Duchess's Contract Marriage

After the ‘Tyrant King’ killed our parents, we managed to defeat him in a coup d’etat. My younger sister became the Emperor and I became the Duke who ruled the beast-infested Northern territory. We took our revenge and my younger sister has become the Emperor, so there’s only one task left – the oath I made to God right before helping my younger sister. In order to fulfill 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?”