My guess is that you need to dispose of the Matron Mother and the demon she summons during the ritual. I think the gates of Ust Natha then open up. I'm not 100% sure on this though. The best thing to do would be to reload before you got discovered, so if you still have an old save game I strongly suggest you go back to that.
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Formidable Flirty Fishy Sorcerer of the HADB