The Moondarkers are a gang of criminals who set out to try and sink ships and steal their cargo. They appear in the story 'Purple Prose' in Ranger's Apprentice: The Lost Stories. Moondarkers usually raid at night, in a new moon, hence the name 'Moondarkers'.
Their method of sinking ship use a false beacon and fires, which imitate a town with a harbor, so that ships will come towards it only to find themselves beached or sunk. The Moondarkers promptly rush onboard and try to kill everyone and steal the cargo before making away into the night.
The village near where the Moondarkers work usually does not mind their presence, as the moondarkers leave the villagers some resources such as timber, ropes, and canvas left over from the ship, which were all greatly welcomed by the poor villagers who have very little.
In Purple Prose, when Will and Halt recieve word that Moondarkers are operating in Araluen in an area near Celtica, the two Rangers set out to stop them, knowing that the Moondarker's next target is a small town named Hambley. They are foiled by the two rangers who use a dye which can be thrown into the fire to alert the people aboard the ship.
Before the Ranger Corps were reformed by Halt and Crowley, the Moondarkers were a scourage on Araluen, but once Crowley and Halt appeared on the scene they experienced the consequent wrath of not only the law but the two Rangers. As Halt said to Will: "You can still find their graves along the coast here".
The man that Will fights put up a tremendous struggle, and was very skilled with an axe. This indicates that the moondarkers either gained much more skill with weapons in the years since they were first seen, or that the leader put one of his best fighters near the big beacon, the important part of the plan, which indicates a certain depth of strategic thinking.