Undercommon

Undercommon is a language known by the half-orc skeletal mage, Chauch-eta. Nothing more is known about this language.

It may be a version of Common or Low Common for the creatures dwelling beneath the earth.

"Undercommon" has an apparent connection to DnD derived generic languages, which apparently can be found in DnD rule books. In other fantasy settings, dark elves speak undercommon.