Language of the Fae Mhor and lingua franca of the Underdark
Originally derived from the Elventongue, the language of the Dark Elves has been significantly polluted by contact with the speech of the Lower Planes, and is now radically different to that of their bright cousins. A completely new set of symbols has been developed to write it with.
As a result of the Light Elves' determined efforts to suppress all memory and record of the Fae Mhor, the language is almost forgotten in the surface world, and very few people even recognize the script for what it is in the unlikely event that they were to come across a sample.
In the Underdark, the Fae Mhor tongue is widely spoken, and in some places forms a convenient "trade language".