The Wall of Destiny is a stone wall located in the Gelfling ruins and holds an ancient Gelfling prophecy. The prophecy stated that at the time of the next Great Conjunction, a Gelfling would heal the Dark Crystal reuniting the Skeksis and Mystics into the UrSkek.
The Gelflings became dismayed when the Skeksis began to take Gelflings, as well as Podlings, as slaves. For once the Gelflings thought of the future and feared that if the Crystal remained unhealed, the Skeksis rule would continue. They lit the fires of prophecy and took counsel from the flames. Seven circles of seven Gelfling (a total of 49 Gelflings) laid on the hilltops all night with their faces to the stars. By ways of their magic their dreams were made stone into creating the Wall of Destiny in the Gelfling city. Careful observation will reveal that the carving has been created by a process involving intense heat, not with the blunt force of physical tools.
The Wall of Destiny showed the Gelflings their quest - to find the lost Shard, and have the Crystal healed by the hand of a Gelfling before the next Great Conjunction. However the Skeksis saw it too, and fearing the Gelflings' success, resolved to stop the prophecy. They created the Garthim and other creatures to aid them. The Skeksis directed the Garthim to hunt down and kill the Gelflings. They used Crystal Bats and Garthim to find and annihilate the entire Gelfling race. However two Gelflings survived the slaughter - Jen and Kira.
When single shines the triple sun,
What was sundered and undone
Shall be whole, the two made one,
By Gelfling hand, or else by none.
Jen, sent on a quest by his dying master to heal the crystal, and Kira, whom he met along the way, discover the prophecy still written on the Wall of Destiny. Knowing his mission, Jen heals the crystal and restores balance to Thra.