UrSkek

The urSkeks were a sapient species which originated in an unknown part of the universe. At some point in history, eighteen of them came to live on the planet Thra and developed a symbiotic connection with the mystical and powerful Dark Crystal. A thousand years before the time of Jen and Kira, during the previous Great Conjunction (the alignment of the three suns of Thra), the Crystal was broken and its released energy caused the eighteen urSkeks to be split into two races: their "evil" ambitious side embodied as the Skeksis; and their "good" wise side embodied as the urRu (better known as Mystics).

When the Crystal was healed during the next Conjunction thanks to the efforts of the Gelflings Jen and Kira, the remaining urSkeks (only eight of them by this point) returned to their original form and decided it was time for them to leave Thra and ascend to a higher plane of existence.