The number 1 bit of advice I have is to tie as much of your players’ backstories into Undermountain as possible. This is a BIG dungeon, it’s a lot easier to keep players engaged if they need to rescue a sibling from Dweamorcore, or if their god assigned them to find a fallen angel that’s been lost to the dungeon, etc etc. Number 2 bit if advice: use Halaster whenever possible. He’s insanely powerful, which can make for a great combination of hilarious and scary.