Shadowspeak

Also known as Darklander.
Odd language that uses the properties of Darklight to propagate, much like how ordinary language uses vibrations (sound), or light (pictures). As a result, it can only effectively be spoken in darklight: even natural pitch-black barely carries the 'sound'. 'Speaking' utilizes magical control more than physical muscles. Very stagnant, unchanging language due to the inherent longevity of the Plane of Darkness. Origins unknown.
Few Cotd actually speak the language, although they all possess the capacity to. Abyssians tend to know it though, being in contact with a race of beings who can't make conventional sounds. Every Shrouded knows the language well, it being a key part of the crawler ritual.
All puppeteers have an instinctual knowledge of shadowspeak. This hints at the idea that it's something more. Shadowspeak contains the ability to convey extremely complex concepts and express abstract ideas; this isn't something you'd expect to be encoded into the instinctive biology of a race. Whether it's the result of an Edict or even a half-dead god like the Deep is up for debate.
Shadowspeak can cause mild headaches in those incapable of perceiving it. This scales up based on its intensity, similarly to the disorientation and pain caused by a loud noise. The effect can be considered a very rudimentary illusion, as the pain comes from Mind-Soul Interface dissonance. Unlike many other sources, it affects the soul side of the interface as opposed to the mental one.
Shadowspeak is, as befits a language primarily used by immortals, rather long-winded. Shadowspeak is like speaking in 1s and 0s to spell out the words you're trying to say using ANSI character codes, while using a thesaurus to make sure that you're using the longest possible word in the given situation.
As a result, groups like the Eyes of Northport have devised much simpler dialects with dramatically higher information density.