Death Preparation Hero

Death Preparation Hero summary:

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 “death preparation journey” to look back on his life.