Undermountain is a massive cavern system built by the mad wizard Halaster on the planet Toril in the Forgotten Realms. It is infamous for being full of deadly traps and creatures, with great rewards for those who can survive.
Hordes of the Underdark campaign
Undermountain is initially accessed through the well in the Yawning Portal, a Waterdeep inn run by Durnan. Once inside Undermountain proper, there are three levels to pass through before being able to confront the source of the drow invaders. Scattered among these levels are the corpses of four of the five henchmen that are available in chapter one—Daelan Red Tiger, Linu La'neral, Sharwyn, and Tomi Undergallows. (The fifth henchman, Deekin Scalesinger did not charge into Undermountain, so remained in the inn and remained alive.) Once cured of death (by raise dead or resurrection) these henchman can either join the player or return to the relative safety of the Yawning Portal.
The first level of Undermountain is dominated by a war between ogres and fey. An ogre mage (in the north) and a fairy queen (in the south) have each raised an army with which to control this level, filling the power void left by the disappearance of Halaster. Settling this conflict by defeating both leaders is one way to proceed to the next level. (The other way is to solve the colored pillar puzzle in the central area of this level.)
The second level of Undermountain features Berger, a flesh golem loyal to Halaster. Berger was fleeing some drow and locked himself in the room with the stairs down to the third level. Unlocking the door, which requires obtaining four chains and solving a color puzzle, is the objective here. Other residents of this level include rakshasa to the east, goblins to the north, and a drow encampment to the south.
The third level of Undermountain is dominated by the drow. This is where Nathyrra first makes an appearance, initially offering guidance on how to more easily overcome various drow encampments, and later offering to directly help save Halaster. The "central" part of this level is populated by formian slaves (who will rise in revolt if their queen is freed), the overseers of whom have established an encampment obstructing passage to the northern part of the level. In that northern part is the main encampment of the drow, guarded by ballista, blocking access to the western part of the level, and containing the items stolen at the start of the campaign. The western part of the level is where Halaster has been held prisoner; freeing Halaster ends the involvement of Undermountain in the campaign.