One thousand years before the beginning of the movie, during the Great Conjunction of the three suns, the urSkeks cracked the Crystal of Truth, and split into two races; the hunchbacked, gentle beings known as the Mystics and the vulture-like tyrants known as Skeksis. In that moment the Crystal became the Dark Crystal. The Skeksis then drove the Mystics from the castle where the cracked crystal resided and instituted a reign of terror over their world. Their wrath was particularly directed toward the elf-like Gelflings, due to a prophecy that promised the restoration of the crystal and the end of the Skeksis' power. The prophecy, rediscovered in an ancient Gelfling city, read:
“ 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. ”
(Above taken from this page... WARNING: link contains spoilers).
I've seen this movie a few times, but it was on TV recently, so I watched it again. After all, it's one of those movies that I just can't help watching several times. It really is a fantastic movie! Seriously, if you haven't seen it then you need to!