I was going to sleep last night and reading through the monster manual and the room with the sand and the wind and the illusions seemed awfully similar to the area around a brass dragon's lair.
-Tracks appear in the sand within 6 miles of a brass dragon's lair. The tracks lead to safe shelters and hidden water sources while also leading away from areas that the dragon prefers to remain undisturbed. (the party couldn't figure out what the tracks were or where they were leading. They did find a pool of water though.
-Images of Large or smaller creatures haunt the desert sands within 1 miles of the dragon's lair. These illusions move and appear real, although they can do no harm.
The actual lair itself is categorized as having a strong wind about it as well as sand.
Adam described to Raziel that the creature was rumored to be seen in the desert and used illusions to protect itself. It certainly could be something else, but I can't help but wonder (if it is a dragon), how it fits into the plot and how it got into a mountain. Though I suppose perhaps it was a friend of the rock gnome king before he became corrupt, or just happens to be there.