The Mystics - also known as the urRu - are one of the two species which was formed from the previous dominant race of Thra: the ancient urSkeks. A thousand years ago, the urSkeks shattered the Dark Crystal and their race was divided in two: their "good" sides personified in the form of the Mystics and their "evil" sides embodied as the Skeksis. Each Mystic is linked to his-or her in the case of urSan-respective Skeksis and vice-versa, so that when one dies the counterpart perishes as well.
The Mystics retained the natural wisdom and benevolence of the urSkeks, but none of their efficiency or ambition. They are shaggy hunchbacked bipeds, bird-like and at the same time slightly sloth-like, with a long tail and four arms. While the Skeksis became the ultimate masters of Thra, the Mystics formed a simple and peaceful, yet mostly cryptic society.
The Skeksis and Mystics have never been a numerous species: by the time of their creation there were eighteen of them; with eight remaining until the time the crystal was healed and they merged back into urSkeks.