The Garthim were the enforcers of the Skeksis' will. There were large, black-carapaced insectlike creatures resembling beetles with large crab claws. Although the General had created the Garthim, each individual Skeksis, over the thousands of years their reign lasted, boasted that they had created the creatures.
Predominantly mindless, the Garthim would stand like sentries through the castle, and would move only to attack intruders, or if given explicit instructions from the Skeksis. Their lifeforce was apparently tied to the Dark Crystal itself, as their eyes shone a luminous purple.
The Garthim were used in the genocide of the Gelflings, and later to raid Podling villages and bring captive Podlings back to the castle, so their life essence could be drained. When Jen healed the Crystal during the Great Conjunction, restoring it to its original, pure state, the Garthim died and fell apart revealing they were just empty shells.