hm, thought has JUST come to me, so at the moment it's in its rawest form.
You are a slave (of any race) who is incarcerated in Ust Natha (that WAS the Drow city in BG2, right?). Anyway, when the char roughs stuff up by going through the city, you escape in the madness and make yourself throught the underdark. At the begginning you pick up a weapon of a dead drow, and maybe band together with some fellow prisoners. Inevitably, you would come to the surface, but it would be nice to play in the Underdark for a while.
Oh, and for the people who will, of course, say "But what if you slipped silently out of Ust Natha, or just went to the surface and didn't bother with them" just remember that BG2 assumed that you knew Jaheirah, Khalid, Minsc, the witch, Viconia, and countless others. Just by assuming that you killed a few drow is not as big a jump than that
