Svirfneblin Son Missing - Baldur's Gate 2 Side Quest
The Svirfneblin Son Missing side quest begins by speaking with Therndle Daglefodd in the Svirfneblin Village located in the northern portion of the Underdark. For this quest Therndle asks you to locate his missing son but he doesn't really give you much detail on exactly where to find him.
You can find Bedlen Daglefodd, Therndle's son, in the Spherical Prison in the central portion of the Underdark. A picture of the prison I am talking about is shown below, it's extremely easy to miss as you run through the area since it blends in with the environment. This Spherical Prison is part of the Victims of Imprisonment unmarked side quest.
How this prison works is there are 6 different NPCs that it can free; to free an NPC from the prison you need to click on a small black box on the Spherical Prison. Mouse across the prison and when you see your cursor change to two arrows in a circle pointing at each other you'll know that you've found a spot you can click on.
Bedlen Daglefodd is in the 5th facet from the left. As I described before, the facets on this prison are the little black boxes surrounding the Spherical Prison that you can click on. All you have to do is free Bedlen and he'll return to the Svirfneblin Village on his own... However there are a few different outcomes for this quest depending on whether or not you've spoken to Adalon yet and are under the effects of the Drow Illusion.
If you're under the effects of the Drow Illusion then you'll have a hard time getting Therndle Daglefodd to believe that you're the one who helped free his son. For more information about this check out the wiki, though I consider the wiki to be confusing about this matter. It says you need a high "reaction" to convince him but I think what they mean is Reputation.