Dreamsign is the sign language used concurrently with Dreamspeak, the language spoken and signed by the Children of the Forest
Averno Bible Description
The children of the forest have their own language that they use to communicate. It has its own alphabet, as well as a sign language that is used concurrently with the spoken language (sometimes you just don't want to talk in the forest). There is no direct translation of its name in English, but the closest equivalent would be Dreamspeak*. The first class of the Department of Dead Languages found Dreamspeak scrawled on the walls of the abandoned grain mill when they arrived. Over the course of the first semester, slowly but surely, the students began speaking It to each other. No one entirely understands how they know it, or how they learned it. Since then, Dreamspeak has echoed down the halls of the DDL as much as Latin or Greek.
*There is a school of thought that Dreamspeak is comprised of the ineffable quality of thought, dreams, whatever language the gods talk in, etc.