A couple of questions!

1) If I leave the Underdark can I get back in?

2) How do I leave the Underdark?

3) How on earth do I beat the demon?

Thanx

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).

[Zoratorak]

answers (spoilers)

1. you can go back
2. go to underdark city first and finish that quest
3. to kill the demon ... well... protect yourself from confusion, held spell and have a +3 weapon ITHINK by your side..

ps: another simple solution?? set traps and lots of traps b4 he even comes out..(youll waste a couple of days though)

[Lord of Alcohol]

You can go back in the same place you came out. I traveled back there looking for a lost vorpal part(lost-gone-poof) but I was attacked in the Drow city. Not hard to kill but I left, I was trying to finish up the rest

1. Yep.
2. Ignore everything and make straight for the kuo toan dungeon, the exit's in there. You can also do it the long way by killing the daemon, talking to Adalon, sneaking around Ust Natha, etc. I think you can also leave and come back to do all this later (say you need to make a quick dash to your thieves guild to file a tax return), but only if you don't talk to Adalon before leaving.
3. Summon something to tank, preferrably a wizard's eye or a skeletal warrior. Then wail on it with ranged weapons and spells.

[Bahamut]

Wait, wait, wait... can't you just use a Protection From Evil? Will it help?

By all rights it should work against a balor, but it never did for me. It does have a chance of failure, mind.

[Bahamut]

Yeah.... only immune to its attacks presumably. But him throwing confusion is all too much huh...