Undercommon

Undercommon (also Deep Speech) is the trade language of the Underdark. Almost every intelligent Underdark creature speaks Undercommon. It is a combination of Drow language, Dwarven, and surface Common, and even has some Goblin language and other "lesser" words thrown in, but it is actually based mainly off the Drow language originally. This language was originally created by slaves of the Drow to communicate between each other as slaves of various races began interacting. It eventually grew to the point that it became a trade language within the "lesser" Underdark races, since so many could speak at least some of it.