The mission will not be available until after you amass a certain number of Rank Points obtained by trading stacks of crystals of any type to any of the War Warlocks (character with the initials W.W. after their name) located at the city gates.
Trading 1 earth (or similar) crystals (not a stack, just 1) will unlock this mission.
Trading 12 crystals will cap your rank points for Rank 2.
Once inside, go to G-6 on the upper map and take the right tunnel to get to lower map.
There are several aggressive mobs on the way, Goblins which detect by sight and Maze Scorpions and Ghouls which detect by sound. Once in fossil room, the active mobs all detect on sound only. You'll need to cancel your Invisible to examine the Fossil Rock, but you can keep Sneak.
Head to H-5 on the lower map
There are four Fossil Rocks in this room. There are also two Fossil Rocks in the large room on the same map at I-8.
Go West from K-9 and take the path north and follow the western wall.
At I-9, take the West tunnel at the 3-way intersection. Follow the tunnel Northwest - again, follow the western wall to avoid the skeletons.
It's recommended to actually enter through the cracked wall at (G-9) as it's safer and does not require Sneak or Invisible. After entering, go north in the large room, then follow the path straight ahead. Eventually you'll make it to the Mahogany Door.
There are reports that the Cat Burglar has her paws on a "Book of the Gods," a restricted book that was lost twenty years ago. Assist the Opistery in investigating the reports, and attempt to retrieve the priceless, but dangerous, tome.
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