This is a summation of episode 21 of the Landscape season 2.
The group descends into the Lost Dinos' excavation, hoping to catch up to MijMaj. They find themselves exploring a strange subterranean cavern system, within which is an abandoned Kuo-toa village (it's occupants slaughtered and left in an underground lake), and a strange network of finely constructed stone tunnels. Within the more well built tunnels, the group finds a chamber filled with the bodies of dead Drow, seemingly burned alive.
While examining the chamber, a tiny laugh echoes through the dark hallways, and the door of the room slams shut, awakening the mass of skeletal undead, and the demonic Immolith residing within the charred effigy of Lolth. The group outs down the Immolith, and the body begins to push out a noxious gas, slowly filling the chamber. Reginald, with his friends cursing over his shoulder to hurry, manages to findangle the mechanism controlling the door, and the adventurers spill out into the hallway.
Once they've recovered, they explore a little further, and discover they have (or rather MijMaj has) discovered a secret rear entrance to a Drow palace. They take a janitor hostage, keeping him from alerting guards to their presence, and raid what appears to be the Queen's bedchambers, stealing - among other things - her crystalline crown.
They finally leave the janitor locked up in the queen's closet, and exit the Drow built halls. Following the cavern system deeper, they turn a corner to find a male Drow reading a book by lantern light. He looks up and see's the strange and exhausted crew, and drops his book in astonishment.