I'm a Retired Demon King. You Said I Was Just Born?

Seymour traveled through time and space, becoming the most absurd and powerful demon king in demon history. However, he arrived too early. It wasn't until he encountered Celia, a challenger in his later years, that he realized he had traveled to the world of Fulilian. Even the demons' near-eternal lifespans are drawing to a close... One day, the aging demon king, Seymour, retired, intending to find a secluded place to spend the rest of his days. He encountered a disobedient demon slaughtering humans, and despite his aging body, he struggled to defeat it. Amidst the ruins, he discovered a little orange-haired girl—"One year old, just born, you encountered the progenitor of human magic, Volame. Your mission is to survive her." Seymour looked at the system he had been waiting for a lifetime. And the little girl, still a child, barely learned magic, lay dying on the ground...