Ryoumin 0-nin Start no Henkyou Ryoushu-sama

The Frontier Lord Begins with Zero Subjects

Dias finally returns home after decades of war. He's hailed a hero and promptly rewarded with his own domain...which turns out to be little more than empty plains. Population: zero. Dias, who has only ever known battle, finds himself at a loss. How is he supposed to survive, let alone cultivate his territory into a thriving, prosperous dominion when there's nothing but grass as far as he can see? Fortunately for Dias, a horned girl by the name of Alna is about to show him there's more to the plains than meets the eye!

(Source: J-Novel Club)