A massive, pulsating violet crystal, the Great crystal was located within the Great Temple on Korriban and contained the spirits of Jedi Masters who had been defeated by the powers of the ancient Sith. Lost to memory after the Sith abandoned Korriban, the souls within the crystal suffered an eternity of endless torment while the crystal remained intact.
While seducing Jedi Padawan Exar Kun to the dark side of the Force, the spirit of Sith Lord Freedon Nadd brought him before the crystal as a final test of his commitment to the way of the Sith. After Kun commented on the horrible fate which had fallen on the Jedi Masters contained within the prison, Nadd decided to "liberate" the spirits, and Kun, from the light side. Nadd destroyed the crystal, annihilating the Jedi spirits within and causing an avalanche that would nearly kill the young Exar Kun.