The urSkeks were a sapient species which lived on the planet Thra and had a symbiotic relation with the mystical and powerful Dark Crystal. A thousand years ago, during the last Great Conjunction (the alignment of the three suns of Thra), the Crystal was broken and its released energy caused the eighteen remaining members of the urSkek species 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 at 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 their world and ascend to a higher plane of existence.