Chthonic (/ˈθɒnɪk/)

Chthonic is an infectious and dangerous language that lurks in ancient textbooks and old ruins. Once exposed to it through written works or hearing it spoken at length, learning it is unnaturally simple. It is spoken by demons and some abberations. Very few are aware of the existence of Chthonic, even among well-studied scholars.
Chthonic is purged wherever it is found by the Inquisitors, also known as the Order of Utterance, an esoteric and secretive department of the Bookwardens allied with the archaic knightly order.

Symptoms of Infection

Many experience headaches shortly after exposure. Continued exposure leads to headaches clustered in the front of the mind, insomnia and terrible nightmares. Should the infected individual choose to pursue learning the languages, the headaches give way. Advanced learners may struggle to communicate in other languages.
The blood of Chthonic speakers is significantly darker than regular blood. This darkened blood is a tell-tale sign used by the Inquisitors to determine infection.
Chthonic infection is often mistaken for Soulsickness, as they are frequently comorbid.

Exhumation

The only known way to unlearn Chthonic is through a secret rite known as Exhumation. Those who have gone through Exhumation are shells of their former self, and must relearn everything from walking, speaking, to basic hygiene.

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I am sad to report to you that Ferdinand Sapire is but a husk of his former glory. The brilliant linguist who developed Imperial Common now can't even go to the bathroom unassisted. He speaks, but my four-year-old has a better grasp on language than he does. He understands more than he can say, though, I shall give him that. I swear I saw his eyes glimmer when I told that joke he used to love about the Courdian Horse.

— A letter home, Mordecai Mallum