As a Reincarnated Aristocrat, I'll Use My Appraisal Skill to Rise in the World 4 (manga)

Now joined by his politically savvy fiancee, Ars continues striving to turn his weak domain’s fortunes around with a little help from his talented vassals. However, everything changes overnight when Ars’s father, Lord Raven, is struck by a sudden illness and the duke of Missian is assassinated. With Lord Raven out of action and war threatening at Missian’s borders, is Ars prepared to take on his father’s mantle?