Here is my situation:

In Ch 6, the Underdark, I have completed the whole dragon egg swap and ritual. I now have the real Adalon eggs, Phaere and the mother are dead (by the demon) and I have escaped. In returning to the Underdark map, I went to the entrance to Adalon's cave in order to return the eggs and complete the quest. However, when I get the stairs icon to enter her lair, a dialogue box appears saying something to the effect of an impenetrable darkness surrounds you and it is impossible to proceed (or something like that).

Is this a bug? Any help would be greatly appreciated.

It sounds like you may have lost the gem that the gnomes gave you before you visited the Dragon the first time.