Looking for a well-written high-level (10-20) campaign setting in the Underdark from any edition of D&D.

[Quartz]

The Shadows of Istivin mini-campaign from Dungeon has part of it set in the Underdark.

 

[Tormyr]

You might want to check Rise of the Drow by adventure a week when it comes out in December.

https://www.kickstarter.com/project...of-the-drow-collectors-edition-for-dandd-5e-a

 

[muppetmuppet]

There is out of the abyss which is good for giving you some locations in the underdark the party might want to travel to and interact with. But you would not be able to run it as is. The overriding plot would be useful but you might well have to write half the adventures yourself. It is for 1-15 and there isn't enough material in the book for that.

 

[Luz]

If you want official there is the 1970s Gygax Descent Into the Depths of the Earth underdark series D1-3 originally for levels 9-14.

D1-2 Descent Into the Depths of the Earth

D3 Vault of the Drow

There are even already done 5e conversion kits for it which pegs the suggested levels around 12-14.

5e Conversion for D1-2

5e Conversion for D3

Having just run these modules for my 5e group, I'll second this suggestion. Written by the man who invented the underdark and drow, these modules have never been equaled. To expand on D1-2 Descent Into the Depths of the Earth, check out Encyclopedia Subterranica it's a community fan-made project and great resource.

Encyclopedia Subterranica