Manga Sayounara Ryuusei, Konnichiwa Jinsei

The strongest and oldest dragon who had lived through eternity allowed himself to be killed by Hero. Just as he prepared for eternal sleep, he realized himself reborn as Dran, a villager in the frontier. Working the field, hunting forest animal, and having a warm life has filled his heart with joy he never experienced in his whole dragon life. One day, when he's patroling the forest near his village, he encountered a powerful demon army...