This story is supposed to take place way after the show's supposed end in season five. Since I don't exactly know how it will end, I'm leaving the way Lucifer is able to retire from Hell for good entirely up to you, the reader. All you need to know for the story is that the demons have all returned to Hell (save for Maze, of course) and have not dared to venture back to Earth, even without Lucifer there to keep them in line.