A Mercenary is a soldier who took part in conflicts for private gain, unlike a knight.
A force of mercenaries sent by Agravaine ambushed the Knights of the Round Table as they went through the Valley of the Fallen Kings. They eventually managed to capture Merlin who was brought to Morgana and turned into an assassin to kill Arthur (A Servant of Two Masters).
Interestingly, the mercenaries looked like they worked for Morgana, although she had nothing to pay them with. Whether they worked for her because of fear is uknown.