***SPOILER***
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You are in the Kuo-Toa lair. The big door guarded by the Drow is the exit to the Underdark. You should *not* go through these doors until you complete the Drow quests in the city of Uth Nasta. I believe that the "altar" is a statue of a Gorgon? If so, put a live sacrifice on the altar and see what happens.
Save your game before you do so, though!