The Skeksis made the Garthim from the memories of sea creatures in their first world, quickened by the light of the Dark Crystal, through the skill of skekTek the Scientist and the power of skekUng the Garthim Master. The strength of the Garthim was unbelievable, but their stupidity was incalculable. They lived in caverns beneath the Castle and left only to plunder the surrounding countryside, from which they returned with slaves for the Skeksis. Their sole purpose was to destroy all they found. The Gelfling understood this and lived in fear of them; the Pod People never understood this, and never realized they had any connection with the castle from which no Pod slaves returned to tell their story.
As soon as the Garthim appeared, the creatures of the World turned against them. Foremost among the enemies of the Garthim were the Landstriders, who provided swift passage for the Gelfling. The beasts allowed themselves to be tamed to be ridden and subjected to the will of the rider. Destruction of the Garthim became their fiercest joy; their speed, their slashing blows, would leave a Garthim crippled before it knew it had been attacked.
The Skeksis bred the Crystal Bats to carry lenses of artificial crystal that sent images of all they saw back to the Crystal in the Castle. At first the bats flew only in the dark, but the Skeksis bred from the bolder and hardier strains, until no corner of the World was unknown to them. And when the Skeksis could direct the Garthim with their all-seeing crystal eyes, they made an end to the Gelfling.
Spyeyes were small bug-like creatures that the Skeksis attached to prisoners to force them to serve as unwilling spies. The Gelfling Lahr had spyeyes attached to him after he was captured in the Castle on his quest to free imprisoned Gelflings. After his escape, Whouf, the dog-like creature who helped him herd mounders, ate the spyeyes off him, which freed Lahr from their powers.