I've been thinking about my new group's adventures in Waterdeep, and one thing I need to have ready is the possibility of their heading into Undermountain. Almost all my resources are for Waterdeep itself, and I own nothing of Undermountain. But then, its long and varied history in setting feel like permission to me to make it my own, putting almost any and everything in this sprawling megadungeon. I'll start with In Search of the Unknown. I really like module B1, and can easily imagine Zelligar and Roghann as heroic adventuring partners of Waterdeep who disappeared years ago, leaving behind their lost stronghold Quasqueton in Undermountain.
So, what would you adapt from old modules to put in Undermountain? I'd love to hear.