You awake in the Yawning Portal Inn, the most famous inn in all of Waterdeep. The inn gets its name from the 40-foot well that thrusts up from Undermountain below into the center of the common room. A waist-high cobblestone wall helps keep the inn's patrons safe from a perilous fall into the depths. Above the well is a pully system the size of a Halfling. The pully's massive rope hoists would-be treasure seekers up and down the well into the topmost level of Halaster's Hold: The Dungeon Level.
The area gets its name from the practice employed by the Lords of Waterdeep to drop convicted criminals into the halls as punishment for their misdeeds. What is known is that this first level of Undermountain receives the most traffic. Most adventuring types fail to penetrate into the deeper sections of the massive complex. It is rumored that there are hidden entrances and exits to this level in the city's sewer system and connected to several of the city's noble houses. What you are certain of, however, is that this is the easiest and most well-known of entrances.
The inkeep, a massive bulk of a human named Durnan, is the strong silent type but as you prepare to venture forth into the darkness he says: "Tymora's luck be with you. Ye'll surely need it."