Abyssal
Abyssal is the language of the Tanar'ri (demons), spoken primarily in the Abyss. The origins, vocabulary, and history of the language itself is a rare knowledge, often kept closely guarded by those who can speak it - mortals and immortals alike. There are no commonly known written records of the language in existence. The "common knowledge" is what can be learned by those... unfortunate enough to hear it spoken.
The language itself is as foreboding as the tormented native speakers are. The dark language is vicious, the essence of demons emitting from the forsaken language when it is spoken. While the reaction of those who hear it will vary, the words can carry emotion with them... a void or a pit, draining out what ever other feelings that are not defined by what is being spoken.
Some say if you stare into the Abyss long enough, the Abyss stares back at you... just imagine speaking it.