kengr

Earthdawn did something thematically similar. The magic level in the world became too high, and demons and Lovecraftian terrors poured through, so the sentient races of the world hid themselves in underground cities, caverns, behind magical barriers, etc. etc.

Generations later, the magic level has decreased - but not as far as it should have. Adventurers explore the new wilds, and sometimes find remnants of hiding places that didn't survive... or sometimes worse, that did.