Trivia: The actors inside of the Skeksis had television monitors inside of their costumes, so that they could see where they were going.
Trivia: The landstriders - large quadruped beings that are able to run fast and for vast distances without tiring, sometimes used as mounts - were actually professional stiltwalkers who used extensions on their arms and legs and were in costume. Wires removed in post-production helped hold them up and prevent serious falls.
Trivia: The Skeksis may seem to have different genders by their voices and behaviors, but are in fact something of both male and female genders. Jim Henson thought that having a world where gender was more ambiguous would be more interesting in general, and for Skeksis, more disconcerting.
Trivia: The Garthim costumes were so heavy and hard to get in and out of that oftentimes the costumed actors were suspended on a rig to allow them to take breaks between takes according to The Making Of The Dark Crystal Feature on the DVD.
Trivia: Augra, the horned being with curly hair, is the guardian of Thra who was initially possessing both male and female attributes-but most of the male part of her was lost while viewing the Universe. She still sports some whiskers on her face to show that she's still not entirely either gender.
Answer: The skeksis and the beings of light are the same, each being a different side (good and bad) therefore it was there evil side (the skeksis) that struck the crystal.