From Wasteland Scavenger to Galaxy Master

This is a world where "low-demon apocalypse" and "high-demon ruins" coexist. Surface world: Magic is exhausted, technology is backward (similar to the Middle Ages), humans are huddled in a few high-walled cities, and resources are scarce. The lower world (cave) is full of huge ruins and relics left by ancient civilization (highly developed magic civilization), full of dangerous monsters and unstable magic environment. Core contradiction: The world barrier is damaged, and a "tide of the void" comes every few hundred years. The last tide wiped out the previous civilization.