Since I haven't finished the dragon fight yet, let me at least answer this.
Potential ingredients in their underdark crawl include:
- blind, psionic, albino, cannibalistic halflings
- ghoulish and non-ghoulish mindflayers
- some up front and personal time with deific representatives
- A very difficult dilemma
I don't want to give too much away. But really, who doesn't love blind, psionic, albino, cannibalistic halflings? The big challenge in this sort of adventure is to make it stay interesting. The problem with the 2e Night Below boxed set, for example, is that the middle bit got boring. Just too much of the same old, same old. I'm going to try and avoid that. We'll see how well I do.
For instance, you'll notice that there aren't any ghouls in Mrid. That's because the ghoulish commanders thought, "Hmmm. Every time our armies meet these people, hundreds and hundreds of our troops die. Whaddaya say we try to avoid them until our assassins can take them down?" Thus, you won't be seeing as many large-scale battles for a while, unless the PCs catch up with an army that is trying to be somewhere that they aren't.
To make this work, I will gloss over the boring bits ("You travel for 4 days.") It should be a theme of epic, fantastic exploring, mixed with occasional vicious danger and clear signs of what the ghouls have done. It should be very clear that these are Bad. And they should wonder where the heck all those ghouls have gone...
But don't worry. The undead are working on something important. After all, the notion that the ghouls could wipe out all living creatures by themselves - religious quest or not - is absurd. They must have something else up their rotting, slimy sleeve.
Following this adventure, they'll have to deal with the aftermath of what the White Kingdom has done. They'll have to deal with the aftermath of what the Church of Aeos has accomplished. I'll probably toss some planar adventuring in there. And Knight Otu's correct; they'll probably want to find out why the king's son is missing, and why they can never remember that fact.