The Skeksis

The Skeksis are the rulers of another world, which homes their most valluable piece of treasure, The Dark Crystal. These creatures are birdlike creatures, added with raptor instinct, each wearing a different set of robes to show their position at their castle. They onced feared that the crystal would be healed by the Gelflings, causeing them to combine with the Mystics, to become one. So they ordered the soldiers, The Garthan, which are crab like creatures, to exicute them, only to miss two survivors. They were defeated when Jen, healed The Dark Crystal, & The Skeksis, along with The Mystics combined into one being.

Origins
In the other world, The Mystics, & The Skeksis were once one being, however, through their arrogance, & greed the Cryatal of Power, lost one shared when it cracked separating them into becoming, The Skeksis, & The Mystics. One thing that surprises them is that even though they were separated, they still feel the same pain. The Skeksis, are the dark side of the same bings, while The Mystics are the light side. After the unification, & healing of The Dark Crystal,  The Skeksis, went to the underworld, only to meet fellow villains that were defeated as well, they teamed up, & now work together to end the Adventure teams.

Trivia

 * The Skeksis became Pooh's enemy in Pooh's Adventures of The Dark Crystal.
 * The Skeksis will become Aaron's enemy in Aaron and The Dark Crystal, at the end of the movie, The Skesis will join The Kingdom of Darkness.
 * The Skeksis will become Arik's enemy in Arik and The Dark Crystal.