Mizu Zokusei no Mahoutsukai Dainibu: Seihou Shokoku-hen

Three years have passed since the battle to liberate Knightley, and Ryou, now conferred the title of head duke, is living his best life. He spends his days going between Crystal Palace and the Forest of Rondo, leisurely farming and practicing his alchemy. But the life of a nobleman proves troublesome when he's ordered to join a diplomatic delegation headed to the faraway Western Provinces. As if that's not enough, the imperial representative is Rupert VI, his former foe and the previous emperor! Even worse, when they finally reach the Western Provinces, they find that the lands are teeming with malevolent creatures like vampires and demons. Despite their mutual distrust, Ryou and Rupert VI must work together to protect the delegation from the monsters and plots lurking in the Western Provinces.

(Source: J-Novel Club)