Croesus
Adventurer
Should this campaign be run linearly in line with Chapter progression? It seems as though the characters could very conceivably choose to go wherever they want at times and end up completely off script so to speak. My biggest concern would be them entering an area(s) that have creature encounters that are too far above their level but I suppose this can be adjusted to suit need. Secondarily I would be concerned that if they got too far away from book chapter progression they might miss a vital piece of information or item required to move past a certain point but I suppose we could just move this to a place that suits the needs of the time as well?
Another possible pitfall is that the party may simply decide to leave. Long before the end of the module, they will be able to escape the Underdark. Depending on the characters' motivations, they might simply decide not to return! We had this very discussion in my group, with two PCs arguing that it was too dangerous, that likely survival and rewards would be better on the surface. From a character perspective, this made perfect sense. (From a gaming group perspective, it would be rather rude to walk away from the module the GM has spent so much time and money running.) Because many of the PCs were not "heroes" in the traditional sense, we had to fudge such considerations and return. Some groups might not do the same.
So try to setup the beginning so the characters will want to stay and save the day, for whatever reasons. Possible approaches could include
- input during character creation
- watching to see which NPC(s) the players like and making that NPC the one leading the resistance to the demons
- tying the defeat of the demons to something the character really wants.
One way or the other, you'll need to encourage the characters - and, therefore, the players - to ignore their instincts for self-preservation and take on demons.
This might not be an issue at your table, but wanted to mention it, just in case.