The Head Thief is the leader of a small band of thieves that operate in the Kandarin area, just south of Ardougne. The thief and his group are involved in the Monk's Friend quest, where it is revealed that they have stolen a Child's blanket from one of the monks' sons. When infiltrating the thieves' lair to retrieve the blanket, the Head Thief will appear and attack, though you do not need to fight him to complete the quest.