The mission will not be available until after you amass a certain number of Rank Points by either trading a single stack of crystals of any type to the War Warlock (character with the initials W.W. after their name) located at the city gates, by killing monsters while Signet is applied to your character, or a combination of the two.
Once inside, go to H-5 on the lower maze map (map 2). 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.
Examine a Fossil Rock and get key itemLapis Coral.
There are 5 Fossil Rocks to examine, one will give you the Lapis Coral. There are several aggressive mobs on the way to this room, 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.
Go to the northwest corner of the ring-shaped room at G-8 and into the room with the Ancient Magical Gizmo. There is a cracked wall here; go through the cracked wall and then proceed to the Mahogany Door.
NOTE: Walking through the cracked wall will take you to a new map of Beetle's Burrow. The cracked wall at G-8 will lead to the new map at K-7. Go Southwest - there are two paths, but you can take either of them, as they both converge at I-9. At I-9, take the West tunnel at the 3-way intersection. Follow the tunnel Northwest - again, take either path, as both converge at your target, the Mahogany Door.
After the cut scene, head to the House of the Hero in Windurst Walls (G-3). After the cut scene, go back to Tosuka-Porika to finish the mission.
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.