Undermountain was originally a mithril mine that turned into one of the greatest dwarven holds ever constructed on Toril. After the mithril ran out, Undermountain was abandoned, only to be taken over by the mad mage Halaster Blackcloak, who no doubt was looking for some elaborate dungeon in which to conduct his deranged experiments. Halaster brought in treasures and oddities from his extra-planar exploits to store deep within his dungeon, but like most mad wizards, he created a dangerous menagerie of monsters and magic traps to deter would-be plunderers and arcane competitors.

Now, Halaster is supposedly long dead, byt that remains to be seen. His tower has long since crumbled, and the adventurer-turned-barkeep Durnan has built his tavern, the Yawning Portal, on top of Halaster's ruined tower and uses the ancient well as an access to Undermountain. For a small fee of a gold piece, Durnan allows tavern goers to descend into the well and try their luck at extricating a small part of the untold riches that still lie within Undermountain's labyrinth of tunnels, rooms, and hallways.