Unspeech

Unspeech is a mysterious and very supernatural language of unknown origin. It might predate Mankind and be as old as the universe itself. Some speculate that like other Higher Languages, it is the human approximation of the languages used by the Powers-That-Be. If so, then it paints a rather negative image of the entities for whom it's a primary language.
Any attempt to meaningfully describe it is a waste of time and effort, for unspeech is a language that's undescribable more than occult average. It is a language spoken with particular qualities of silence. It can only be written down by defacing letters of other languages, and read only with your eyes closed. if those who do learn some of it are to be believed, nothing good was ever written in it, as unspeech lacks words for things and concepts such as creation or love. It can only speak of entropy, the inevitable end of all things, and human (or not) suffering.
Only some of the most powerful of grimoires are written in this unholy language, almost all of them devoted to the Ancients. In the end, if you are an Initiate and you stumble upon something written in this language, it's considered a very good idea to let it burn. Without trying to understand it.