Near Infinty and Infinity Explorer can help out, as can the IESDP project.
In the IESDP resource, looking at
"Areas
BG1: TotS || BG2: ToB || IWD1: TotL || PST"
You find a link to IWD1: ToTL which happens to include the main IWD areas, among which is AR6014 Dorn's Deep - Bandoth's cave. Your problem lies in the area leading to this. (I have IWD1 but I have never installed or played it.)
Check the bcs script files for the area concerned.
There is a walkthrough available too:
Icewind Dale 1 Walkthrough