Ryoumin 0-Nin Start no Henkyou Ryoushu-sama

Dias finally returns home after decades of war. He's hailed as 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. 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.

Synopsis from Anime News Network