Ok i went back and played that part of the game and there is no way to reach Thyaast without a rogue. You can not smash open the trapped locked door just north of the her bedroom either. Beyong this door there is a series of rooms that led to thyaast.
If you what to find her you must recruit a rogue in your party. Anyway you can move on if you like since Thyaast quest is a side quest. 
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"When I am working on a problem I never think about beauty. I only think about how to solve the problem. But when I have finished, if the solution is not beautiful, I know it is wrong."
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