The Valley of the Mystics is the isolated valley in The Dark Crystal where the Mystics have lived ever since the the Crystal cracked. Unlike the Skeksis, the Mystics eke out a peaceful life apart from the rest of Thra. They posess all the vast mystical knowledge of the UrSkeks, but cannot make use of it outside of their valley.
It is here that the Gelfling Jen was raised, and from here that he started on his journey to heal the Crystal after UrSu died. Shortly afterward, the remaining Mystics undertook their own one-way trek from their valley to the Castle of the Crystal to be reunited with their other selves, the Skeksis.