You asked Pyonter if he could think of anybody else in the Underdark who might know a way out. He could think of two: the whisper gnomes to the south, and Lum the Mad on the east side of the lake. You decided to first try the one not described as “mad.”
You did interact with them, but they were not especially friendly. In fact, Archimodar almost bit it. They inferred there was a way out of the duergar capital city, but they’ve never seen it themselves. They suspected that Lum the Mad would know, if anyone knew, and sent you gruffly on your way.
So you decided to try your hand with Lum. Things did not go well. He eventually went nuts and in the ensuing battle, you discovered he was a psion, and you killed him. You also discovered, however, a collection of duergard history texts that told you three important things.
(1) There is definitely an exit to the surface in the duergar capitol of Thoduum. Sadly, the only way to get to it is through a duergar puzzle wall.
(2) There is another exit in the collapsed home of a duergar necromancer. The duergar themselves destroyed the necromancer, but perhaps the exit is still there.
Leech explained duergar puzzle walls. The main implication is that you would have to follow the drum’s path to Thoduum, and that means starting in Whurnore and then crossing the underground lake.
This leads to important thing (3). Whurnore was occupied by the spirit of the dead prince given “a gruesome unlife.” And the lake holds an ancient evil. Bummer.