Sayounara Ryuusei, Konnichiwa Jinsei (alpha-manga)

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…