The Skeksis 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 (a.k.a. urRu) and their "evil" sides embodied as the Skeksis. Each Skeksis is linked to his-or her in the case of skekSa-respective Mystic and vice-versa, so that when one dies the counterpart perishes as well.
The Skeksis retained the efficiency and ambition of the old urSkeks, but none of their wisdom and benevolence. Their appearance is skeletal, with light-blue skin and vulture-like facial features. While the Mystics went on to live a peaceful and archaic existence in the valley, the Skeksis remained in their castle and ruled over the planet for a thousand years. They are mostly carnivorous, preparing huge banquets with roasted Nebrie and other native life forms; but are also able to draw their energy from the sun by using the Crystal.
They are masters of the Garthim warriors, the Crystal Bats and the captured Podling slaves drained of their life essence. They have driven the Gelfling race to near extinction, attempting to prevent the Prophecy that stated that during the next Conjunction a Gelfling would heal the Crystal and restore the Skeksis and Mystics to their original state. The Prophecy was ultimately carried out, though.
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.