You track the surviving goblin for several hours along an underground passageway until you emerge onto a settlement. This turns out to be an abandoned duergar city on an underground lake.
A group of myconid have inhabited this village and, in order to stop the goblins and any more humans from following, they set of a cave-in that bloclks your passage out. You are trapped in the Underdark! You eventually make friends with the myconid, and solve a few of their problems, before meeting (on Freezing 24) a human druid named Pyonter. He is charged with protecting the underground ecosystem of the lake, and the myconids specifically. He tells you that the goblins are camped in the largest abandoned city (to the north). He gives you the general lay of the land, and who might be able to help you find a way out. He does not know a way out himself. (He was teleported in, and will be teleported out when he is done.)