Thra is the name of the world created by Jim Henson for the film The Dark Crystal. It is a planet with three suns: the Great Sun, the Rose Sun and the Dying Sun. The main landmark of the planet is the Castle of the Crystal which houses the Dark Crystal itself. When the movie starts, the shattering of the Crystal has caused a dimming of the entire world.
To the southeast of the Castle of the Crystal lies the valley of the urRu, called the Valley of Stones, filled with mist, caves, and springs. Northeast are the Gelfling city ruins, filled with stonework, tiled floors, and frescoes. Near to these is the Landstriders hill and the Podlings' village. Also in this area is a swamp which has animated vegetation and is filled with a crackling static charge shortly after dawn.
Age of Innocence
Shortly after its formation, the rocks and trees of Thra grew restless over their not having the ability to see or speak. In response, Thra created Aughra to be the world's warden and guardian.
Age of Harmony
A thousand trine after its formation, the Fallen UrSkeks arrived on Thra through the Crystal during the first Great Conjunction. Although the UrSkeks desired to return to their homeworld, they could only do so during another Conjunction. Thus, for the next one thousand trine, they assisted both Aughra and the Gelflings in their technological development, allowing the former to study the movements of astral bodies, and the latter to cross oceans and colonize much of the planet.
Age of Division
When the time came for the urSkeks to return home, the darker halves of their natures prevented them from entering the Crystal. Instead, they were split into two new races; the cruel Skeksis and gentle UrRu. The Skeksis took control of The Castle of the Crystal, while the urRu took refuge in the Valley of the Standing Stones.
Age of Resistance
The Skeksis cracked the Crystal, chipping off a shard and thus infecting the world with the Darkening. The Skeksis and the Gelflings formed the Alliance of the Crystal, though the aging Skeksis eventually began to capture Gelflings for their essence. When the Gelfling elders prophesised that one of their own would heal the Crystal and reverse the Division, the Skeksis initiated the Garthim War, aiming to exterminate them all. The species was ultimately cut down to a more two individuals; Jen and Kira. Jen's quest to restore the shard to the Crystal began after the death of his urRu master urSu, who instructed him to find Aughra before the Three Suns met again. Jen took the shard from Aughra, and encountered Kira. Together, they rendered the Crystal whole again, and the urSkeks departed to their homeworld, leaving Thra green and verdant again.
Age of Power
Jen and Kira's rule as caretakers of the Crystal saw the re-emergence of the Gelfling, though their keeping of Skeksis artifacts and abuse of the Crystal maintained scattered pockets of the Darkening in some areas. Their accumulating of offerings to the Crystal prevented its light from nourishing Mithra's Mother Sun, causing the Great Dim which nearly resulted in the extinction of the Firelings.