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As far as the skulls, that is a dirty trick on Argimon's part. Don't fall for it.
One is in the room with paintings and the second one is at the end of the main corridor (the one you started the floor in). You need to talk with them. After you talk to both of them, you will be able to set invisible one wall (with the Mole Dagger) - this wall is at the end of corridor that leads to the Torture Chamber.
To talk with them, you need to explore that level more.
See if the information here helps: