Mercenary

A mercenary is a soldier who took part in conflicts for private gain, unlike a knight.

Mercenaries made up most of Escetia's forces. (The Tears of Uther Pendragon)

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)