The Great Division was a monumental event in Thra's history, taking place nine hundred and ninety-nine and one trine after the arrival of the Fallen urSkeks. It resulted in the division of the urSkeks into Skeksis and urRu, and marked the end of the Age of Harmony.
The 18 Fallen urSkeks were banished to Thra for failing to overcome their darker natures. They entered Thra through the Great Crystal, during the planet's first Great Conjunction. Attempting to redeem themselves by influencing the technological and intellectual development of Thra's inhabitants, namely Aughra and the Gelflings, the urSkeks nevertheless persisted in attempting to contact their homeworld, though without success. Realizing that they could only return during Thra's next Great Conjunction, which would take place nine hundred and ninety-nine and one trine after their arrival, the urSkeks built the Great Observatory for Aughra, who assisted them in measuring the precise moment when the Conjunction would occur.
Ten trine prior to the second Great Conjunction, Aughra became increasingly isolated from Thra's inhabitants as she assisted the urSkeks in their plan. They harvested glass from the walls of the Castle to make a giant network of mirrors around the Crystal, which would focus the rays of The Three Suns onto each urSkek and purge them of their dark nature, as well as open a portal for them to cross back into their homeworld. Aughra also constructed Star Staffs from the Castle walls as the final tools to focus the Crystal's light.
As the second Great Conjunction approached, the urSkeks invited delegations representing each of Thra's sentient races to attend the event. The Gelfling delegation included Lady Kel, daughter of one of the great clan mothers, and song-teller Gyr, while the Podlings were represented by Hotha, son of Noy village's clan mother Hakmeena. Joining both delegations was Aughra's son, Raunip, who saw the occasion as an opportunity to demand answers from the urSkeks regarding their past. Arriving at Skarith by Landstrider, the delegations were welcomed by the urSkeks, who provided food, and allowed them to wander the Castle as they pleased. Gyr entered the Hall of Reflection, where he encountered an urSkek musician. The urSkek, upon finding out that Gyr knew an urSkek song, pleaded that Gyr play it. Upon hearing it, the urSkek realised that Gyr was playing the song the urSkeks had previously used to unsuccessfully call their homeworld. Wracked by nostalgia, the urSkek departed from the Hall, its inner light now darkened. Raunip exacerbated the situation by showing his distrust of the urSkeks for being outsiders who were exiled to Thra.
It's not working! I feel nothing! The Crystal abandons us! Our own kind will never come for us! We are stuck among these savages forever!"Dark Heart", 
On the day of the second Great Conjunction, the urSkeks gathered around the Crystal in the Crystal Chamber, wielding the Star Staffs before them. Before the Suns aligned, the darkened urSkek's rage was stoked by Raunip, who mocked it for being the only true individual among the 18. When the Three Suns finally shone down onto the Crystal, the dark-hearted urSkek's fear and anger prevented the other urSkeks from being purified or from entering the Crystal. In the ensuing confusion, each urSkek was split into two different creatures: the Skeksis and the urRu with the two pairs that were originally YiYa and HakHom killed off by a rogue Skeksis and the Crystal cracked soon after.
While the urRu departed the castle, the Skeksis convinced Aughra to entrust them with the Crystal while she traveled the universe via astral projection. But Thra underwent a dark period during the Skeksis' reign that brought about an Age of Rebellion when the Gelflings, who revered the Skeksis, learned of their actions against them and their world. By the time the Division would be reversed in the Third Great Conjunction, the Gelflings had been driven to near extinction by the events of Garthim Wars.
- ↑ Froud, B. & Llewellyn, J. J., (2003) The World of the Dark Crystal. Pavilion Books. ISBN 1-86205-624-2
- ↑ Froud, B., Holguin , B., Sheikman, A. & John, L. (2011). The Dark Crystal: Creation Myths, Vol. I. Archaia. ISBN 978-1-936393-80-0
- ↑ 3.0 3.1 3.2 3.3 Froud, B., Dysart, J., Sheikman, A. & John, L. (2012). The Dark Crystal: Creation Myths, Vol. II. Archaia. ISBN 978-1-936393-80-0