You know nothing of us! Our culture! Our ways! The knowledge we have gleaned from our unity! It is beyond you!
The urSkeks were an ancient race from the furthest reaches of the Uni-Verse, characterized by their tall, luminescent appearance, psychic abilities, and collectivist society. They conceived of everything in triune terms and considered duality a dangerous heresy.
A group of dissident urSkeks, known as Fallen urSkeks, were banished from their home world to Thra at the start of the Age of Harmony for attempting to use their world's crystal in a dangerous experiment meant to divide their light and dark selves.
After a thousand trine of attempting to reform themselves, and influence the cultural development of Thra's sentient races, the urSkeks sought to purify themselves and return to their home world through the power of the Crystal of Truth during the second Great Conjunction. In so doing, each of the Fallen urSkeks inadvertently divided themselves into two separate beings; the violent, materialistic Skeksis, and the gentle, contemplative urRu.
We must master our darker nature. That is what it means to be urSkek.Unnamed urSkek, 
The urSkeks were tall, upright beings which shone with a beautiful internal light which would turn dark when angered. They appeared to be made from distilled light rather than actual substance and possessed powerful psychic abilities, being able to telekinetically construct edifices, levitate objects, heal injuries and return the dead to life. The urSkeks also possessed advanced technology, being capable of creating mechanized astrolabes and living machines gifted with intelligence.
Little was known of the urSkek home world, other than the fact that it was "far beyond the farthest star", that it was born of a crystal like Thra, and that it vaguely resembled the Castle of the Crystal's Hall of Reflection. Other than the urSkeks themselves, the planet was inhabited by Garthim-like sea creatures and Bohrtog's race.
Much like the creatures of Thra, the urSkeks perceived space and time as essentially triangular, and this was reflected in their art and architecture. To them, a surface consisted of three interconnected dimensions, with the movement of one altering that of the other. The triangle represented the unity of three qualities, including active, passive and neutral, creation, preservation and destruction, and God, cosmic and individual. The urSkeks also visualized time as forking paths corresponding to everted triangles rather than a straight line.
Banishment to Thra and the Age of Harmony
The eighteen Fallen urSkeks were exiled to Thra after they committed "heresy" by attempting to use their homeworld's crystal for a grand design their people deemed folly. They never fully divulged the reason for their banishment, though Aughra and J. J. Llewellyn speculated that their belief in the duality of the soul went counter to their race's dogma, which saw reality as essentially triadic in nature, with Llewellyn opining: "What to us can seem simple opposites–odd and even, light and dark, good and evil–are not so in a world where opposites cannot exist without resolution in the third side of the triangle. That the urSkeks should have ignored this and attempted a great work based on simple duality is beyond comprehension".
They were promised the right to return home, on condition that they master their darker selves. The Fallen urSkeks arrived on Thra through the Great Crystal, during the planet's first Great Conjunction. There, they encountered Aughra, who had been badly burned by the intense heat given off by her close proximity to the Conjunction. They healed her, and introduced themselves as "light bringers", promising peace and peerless gifts. Shortly after arriving, the urSkeks sang to the mountain containing the Great Crystal, demolishing it and encasing the Crystal in a crystal fortress.
For the next nine hundred and ninety-nine and one trine, a period known in Thra's history as the Age of Harmony, the urSkeks imparted their knowledge to both Aughra and the Gelfling. To the latter, they gave tools, and the knowledge necessary to tame nature and expand their territories. To Aughra, the urSkeks built the Great Observatory, and taught her how to manipulate energy through the use of numbers. Nevertheless, the Fallen urSkeks yearned to return to their home world and attempted numerous times to communicate with their people without success.
The Great Division
Wisdom and compassion have their roots in the same soil as violence and hate. That the urSkeks did not understand this and violated their nature was the ruin of our World. They looked and spoke like superior beings, but they too were capable of error.
After a thousand trine had passed since their arrival, the urSkeks created a network of mirrors around the Crystal with Aughra's help, intending to trap the light of the next Great Conjunction to re-enter their world and burn out the imperfections in their souls in the process. In preparation for this, the urSkeks invited delegations representing all of Thra's sentient races to watch the occasion. However, one urSkek became wracked by nostalgia after hearing a member of the Gelfling delegation play an urSkek song, and allowed its darker nature to overwhelm it. Aughra's son Raunip, who had always distrusted the urSkeks for being outsiders, used this as an opportunity to confront the urSkeks directly and he mocked the darkened urSkek, whom he nicknamed Dark Heart. When the Three Suns met, Dark Heart's rage prevented the rest of the urSkeks from entering the Crystal and being cleansed. Instead of being purified, the urSkeks were divided into two separate species; the cruel Skeksis and the gentle urRu. A fight broke out, resulting in the deaths of two Skeksis/urRu and causing the rest of the urRu and the delegations to flee the Castle. Aughra remained, and attempted to reason with the Skeksis, but they refused to listen, and cracked the Crystal, chipping off a shard, which disappeared.
Despite the separation, there was still a spiritual link between each Skeksis and urRu counterpart, a constant reminder that they were only halves of the same being. If either was wounded, the other felt the pain and suffered an identical injury. If one died, both died.
The Two Made One
Another nine hundred and ninety-nine and one trine later, only ten of the original 18 pairs remained, with one passing away, and another being consumed by lava. By the upcoming Conjunction, thanks to the Gelflings Jen and Kira, the process that created the two races was reversed and the urSkeks were reformed, and departed for their home world, leaving the Gelflings as the keepers of the Crystal.
Briefly Divided Once More
A hundred trine after the reunification of the Skeksis and UrRu, a Fireling called Thurma once again broke a shard from the Crystal. This somehow returned the surviving UrSkeks to Thra, once more divided; the Skeksis remembered their previous existence as Skeksis but seemingly had no memory of the hundred trine they had spent as part of the reborn urSkeks, believing that they had been "imprisoned" in the Crystal these hundred trine. The Crystal was eventually restored and the Skeksis and urRu merged once more. Leaving Thra once again, the urSkeks took Jen and Kira with them. 
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