Also called the Lost Tomb, the entrance is located on a ledge between the entrances of Imps' Lair and Raze's Flying Fortress. In order to enter you have to have found all four adventurers in the Grave Robbers quest. You meet Arthur Gremory and Laura outside the entrance where you join the four pieces of the seal to unlock the door.
Inside, Arthur and Laura will notice that the chamber is empty, and there's a note from Bellegar, pointing out that the adventurers still have their health.
There are three levers inside the cave; one placed directly in front of the smashed tomb - revealing a chest on top of the pillars, one behind the crates by the stairs - revealing a platform, and one on the wooden platform (up the stairs) - revealing another platform. After revealing the chest and the two platforms, you can jump to the chest to loot (amongst other things) a Scorpion set piece.