In the pre-production notes for the film, the Garthim-Master is "identified by the sound of his spurs striking on the castle floors and his complete lack of subtlety. The Garthim-Master enjoys tearing apart soft Gelflings for sport." The Garthim-Master is identifiable amongst the Skeksis for its rattling armor, yellow fangs, and bellowing voice.
"Strongest of all for brute force, after the Emperor was skekUng the Garthim-Master. Torment was his pleasure, though his urSkeks original had been a healer and continued so in his urRu form. Hidden in that tall, shining urSkeks was one who, ages later, could find pleasure in tearing apart the gentle Gelfling. The urSkeks knew this evil was in them and tried hard to burn it out. The soul driven by strong forces is weak, and the soul without strength is helpless. Wisdom and compassion have their roots in the same soil as violence and hate; the strong being allows them to fight within him, to recognize that his strength of will must control the evil he could do. That the urSkeks did not understand this and violated their nature was the ruin of our World. They looked and spoke like superior beings, but they too were capable of error."
SkekUng was the Skeksis General (sometimes called the Garthim Master). He was the strongest and most violent of the Skeksis since their creation. He created the Garthim and used them as his soldiers to enforce the Emperor's will throughout Thra.
When the Emperor took ill days before the Great Conjunction, the General plotted to usurp the throne. His one and only challenger was the Chamberlain. The Chamberlain challenged him to Trial by Stone... a contest the General won using his tremendous strength. As his first official act as emperor he banished the Chamberlain.
The General-Emperor's reign would last mere days. He grew increasingly impatient with the Scientist's inability to extract viable life essence from captured Podlings, and, when the former Chamberlain captured Kira, an act for which the General rewarded him with his old position back, the General listed to the Scientist that they should drain Kira's essence. The High Priest insisted they kill her instead. The General compromised: they would drain Kira's essence, and then kill her.
After sending the Scientist away with Kira, the General convened the Skeksis in their temple to await the Great Conjunction. In the meantime, Kira escaped and the Scientist was killed. The Skeksis' were never able to successfully attain immortality from the Conjunction, both because Jen restored the shard to the Dark Crystal, restoring it, and also because the Mystics appeared.
When the light from Thra's three suns shone through the Crystal of Truth, the Skeksis and the Mystics combined once more into their original, natural states. The General combined with his Mystic counterpart, Healer to form the UrSkek UngIm.