Kokurou Ou to Hakugin no Niehime: Henkyou no Chi de Saiai wo Eru

Princess Eder of Zels, who has grown up oppressed by the queen and her brothers because she is the daughter of a servant girl, marries the king of the nation that has just defeated them instead of her sister as compensation for losing the war. She marries Ortius, known as the "Black Wolf King." He is a barbarian king who is rumored to have the body of a wild beast and is only capable of fighting. However, on the day of the wedding ceremony, Eder meets a beautiful, tall, black-haired young man with a bright light in his eyes. Their chance meeting soon turns into a situation that shakes the survival of the kingdom...

(Source: Kadokawa, translated)