1) Well, it was definately a bug of some sort but because I was on an I've-completed-the-game-but-want-to-complete-all-quests I returned to the Shadow Thieves Guild to speak to the leader. Well, I managed to get back to Spellhold and used the exit in the asylum to get back into the Underdark. Don't know if you can do this at just any time though.
2) Underdark requires you to speak to a Silver Dragon who will, if you complete a set of tasks for her, grant you leave.
3) However, you haven't beaten the demon yet, and in order to meet this dragon you need an item from the Sverniblin leader. Defeating demons IS hard. First of all I got a character who had a fairly good magical resistance to go into the area with the pit. Now, just north of the pit is an area which only human-sized creatures can fit into. From this area let the demon loose. (Btw, the rest of your team should be far away for the time being). The demon will try to attack your character but can not reach them if you are in the correct place. Instead the demon will cast a series of spells which will effect but not harm your character. Spells as you prolly realise by now have no effect. You need weapons above I think it +4 of +3 to harm the demon. Well anyway, once the demon stops casting spells on your out-of-the-way character you can get your fighters to charge him (preferably hastened and with any other beneficial spells you can cast on them).
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