The Frontier Lord Begins with Zero Subjects: Tales of Blue Dias and the Onikin Alna

After decades of war, Dias returns home as a hero and is rewarded with his own domain. When he arrives at his new lands, however, they turn out to be little more than endless grassy plains without even a single subject.
Literally put out to pasture, Dias is stumped as to how to live any life other than a soldier's. But when a horned girl named Alna crosses his path, Dias takes his first steps to being the lord of a not-so-empty frontier.

(Source: J-Novel Club)