The urSkeks were noble beings shining with an inner light who entered the world through the Crystal. In attempting to perfect themselves, they accidentally tore themselves apart to become two distinct races - Mystics and Skeksis. Only if a Gelfling healed the Dark Crystal before the next Great Conjunction could the urSkeks be born again and the world of Thra be restored. After the Crystal was restored, they left Thra for an unknown location.
There were originally 18 urSkeks that were divided into Mystics and Skeksis. However, by the time they were rejoined, only 8 pairs were left. UngIm is the only urSkek to speak; he talks to Jen after the reunification of the crystal and before the urSkeks ascended from the planet.