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?”