The Old Gods leaving seems rather tame and underwhelming. How about the humans rising up and slaying their god kings. And only much later they realize that they actually did protect them from a great threat.

Druid magic could be treated as people searching for an alternative source of supernatural power after the disappearance of the gods, which they found in the savage and violent aspects of nature. It's a "clean" source of power that is uncorrupted by the influence of the new alien horrors, but that doesn't have to mean it's peaceful and gentle. Instead it's all about blood sacrifice, teeth, claws, and berserker strength. The civilized people may think it savage, barbaric, and primitive, but it works at repelling nightmare beasts and cleansing the corruption of other magic.