Resurrected Demon King summary:

After a long-standing battle between the demons and humanity, the Demon King Luruslia Nord was laid to rest at the hands of a hero. However, his soul continued to wander. After thousands of years, Luru awakened in a world devoid of demons. Moreover, he was reincarnated as a weak human! To search for his disappeared brethren and to understand the meaning of the new "life" he's been granted, Luru sets forth on a new adventure.