Out of the Abyss centers around demon lords being set loose in the underdark, as the party tries to escape the underdark. What's really nice about it is that it does a good job of filling out the Underdark itself, like with challenges and friendlies, resources, etc.

I second having the Duergar being more of an enemy focus, as Drow have been used a lot. Mordenkainen's Tome of Foes does a good job of providing information about the Duergar.