From A Wiki of Ice and Fire
The King's Justice is the king's royal executioner. The duties of the King's Justice is to kill anyone put to death by order of the King or small council. The executions are usually done publicly via beheading. Other responsibilities of the King's Justice is managing the dungeons at the Red Keep and supervising the chief gaoler, chief undergaoler, undergaolers and turnkeys. The King's Justice answers to the Master of Laws in matters relating to the managing of the dungeons.
Ser Ilyn Payne was named to the King's Justice when Robert Baratheon wed Cersei Lannister. His appointment was a gift from Robert Baratheon to the father of his new bride, a sinecure to compensate Payne for the tongue he lost while in service to House Lannister.
List of Known King's Justice
- Ser Ilyn Payne