This shows up on the map as "Lost Dwarven Kingdom of Reknor."
The third level has a fixed layout. After killing the guardian, Golbug the Destroyer, you complete the "Let's hunt some Orc" quest, and begin the "The wild wild east" quest. Harno, Herald of Last Hope will appear and deliver a message to you. There will be a farportal in the easternmost room, which, when activated using the Orb of Many Ways dropped by Golbug, will take you to the Unremarkable Cave in the Far East, completing "The wild wild east" and granting the Sliders achievement.
After returning from the Far East, the backup guardian is Lithfengel, which is part of the quest Back and there again. It does not matter if you kill the boss before or after receiving this part of the quest from Tannen.