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SPOILER!
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Well you're going the right way, and if you found both the Mind Flayer Area in the Sewers (which is in the North) and the place where Haer'Dalis is held captive (which is somewhat more to the west), then you're definitely on the good path. Just venture some more in the sewers, killing some shadows (which give you some XP, of course). Watch out for the Devil Shades, they can call more shadows to their aid.
The place is not that hard to find; just look for some exits in the sewers.