The Great Conjunction is the end of the world...or the beginning.
The Great Conjunction is the event that occurs when the three suns of Thra - the Great Sun, the Rose Sun, and the Dying Sun - align in the sky. The alignment of the suns form an intense beam of light that shines directly onto the Crystal.
A Great Conjunction occurs every nine-hundred and ninety-nine trine and one trine.
The burning light focused towards Thra during the very first Great Conjunction is responsible for creating the Great Shaft and revealing the Crystal deep within the mountain.
When a sun shone down the shaft of the Crystal, all life on Thra could feel the trembling from the Crystal. The Crystal sings quietly at the lesser conjunctions of the two Suns; but at the Great Conjunction, the song of the Crystal resounded through all the rocks of Thra.
During one Conjunction Aughra gave one of its eyes to see the splendor of the Three Suns joining. She laid on the mountain above the Crystal and saw the Three Suns move together, only opening one eye to the intense light, and for one moment she saw the eye of the Three Suns shining down on her. It was during that same Conjunction that the urSkeks opened the door in the Crystal and entered Thra.
The urSkeks came to Thra using the power of the Great Crystal; they came during one Great Conjunction to use the next nine hundred and ninety-nine trine and one trine, to achieve their work. In the heart of the Crystal they had made a trap for light, a net of crystal and golden mirrors. They caught the light that had passed through the Great Crystal, the light that fell to the Lake of Fire deep within the world. They placed carved mirrors into the beam of light, they shone it into the Chamber of Life. During the next Great Conjunction the three suns moved over the Crystal, and the light was focused and sent through the chamber. The urSkeks entered the bright light in hopes of purifying themselves; but as they left the path of light, they become two -- the evil Skeksis and the gentle Mystics.
On that day, the Skeksis, full of anger, struck the Crystal. A shard broke from the Crystal and flew up the shaft, out onto the mountainside. In order to reunite the Skeksis and the Mystics, the pairs would need to re-enter the focused light of the pure Crystal during the next Great Conjunction. However with the Crystal cracked, it had to be healed first.
The Gelflings made a prophecy stating that one of them would need to heal the crystal in order to restore order during the next Conjunction. With only days left before the next Great Conjunction, a young Gelfling named Jen was sent by his master UrSu to heal the Dark Crystal. He succeeded in his quest and the urSkeks were once again reunited.