The shale tablet is an item found in Lamistard's Tunnels during The Curse of Arrav.
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I have been struggling to get precisely under the stone. I've wasted time digging tunnels the wrong way. I seem to have hit some sort of brick structure, possibly a cellar belonging to one of the others. Maybe if I break through it I can get my bearings. It will be dangerous, though - I don't want to be detected.