The Wandering Tongue of dreams. Learning this Tongue allows a speaker to lucidly enter the Dreamlands in their sleep. All others who currently sleep can be found there, but their dreaming simulacra only speak Mnous. Things stranger still lurk in the Dreamlands as well.
A Fluent speaker can speak directly to the subconscious of others to calm their emotions, even while awake. They get a Save vs Discipline to negate, becoming immune until they next sleep.