Shuukatsu Yuusha

Several decades have passed since the Hero defeated the Demon King and brought peace to the world, and the Hero has grown old enough to only vaguely remember his magic spells. When the remnants of the Demon King's army attack the Hero, he decides to set out on a journey again. This time, it's not a journey to save the world, but a "journey to the end of his days" to reflect on his life.

(Source: Shinchosha, translated)