Shuukatsu Yuusha

Several decades have passed since the Hero defeated the Demon Lord and brought peace to the world. The Hero has grown so old that he barely remembers his magic spells. When the remnants of the Demon Lord's army attacks the Hero, he decides to set out on a journey again. This time, it's not a journey to save the world, but a "final journey" to look back on his life.

(Source: Kurage Bunch, translated)