Reddit reviews Expedition to Undermountain (Dungeons & Dragons d20 3.5 Fantasy Roleplaying, Adventure)

I think you're looking for this. You'd have to update basically everything you see, but if you want Undermountain - this is Undermountain.

edit: D&D 3.5 and 4e D&D made conversions of the original, those might also might be an avenue to take.

I'm unfamiliar with 5th edition and any changes they have done to any campaign settings, so I might say something pretty stupid here.

So, why not go for something that has already been done in the FR setting? You could use the Undermountain, located under the mountain range (and the city of) Waterdeep. There is a 3.5 supplement of Waterdeep, and a 3.5 supplement for the Undermountain (and there is also one for 4th edition, but I have no experience with that one). The waterdeep book is mostly flavor, and you can use the undermountain book for the same reason and simply convert whatever you find interesting to suit your 5e needs.