Undermountain

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.

Undermountain is the source of the troubles that frame the start of the Hordes of the Underdark campaign. It is also the setting of the Infinite Dungeons premium module.