Undercommon

The common tongue of the Underdark, Undercommon is a mixture of Duergarim, Drow, and Goblin, the three most dominant humanoids in the deep and dark places of the world. It seems to take most of its grammar from Deep Dwarven, but phonetically sounds closer to Dark Elven, though unmistakably dissimilar. Thus, a creature speaking even perfectly fluent Undercommon would still be unable to understand either base languages.
Undercommon uses Undermarks, a system of simple scratches and gauges similar to Gobscratch in cavern walls that over millennia developed into a coherent script, consistent throughout most of the innumerable passageways and chambers.