The arrival of a mind flayer nautiloid means trouble for any world—and adventure for that world’s heroes!
If encounters are the building blocks of a D&D adventure, then adventures are the building blocks of a D&D campaign, for a campaign is what you get when you string two or more adventures together. A campaign setting is the world in which those adventures take place—both a backdrop for your adventures and a hotbed of conflicts and personalities that can inspire and drive adventures.