Ser Ilyn Payne is a mute knight and the King's Justice, the royal executioner, to Robert Baratheon and his successor Joffrey Baratheon. He executed Eddard Stark for treason.
Ser Ilyn Payne had his tongue removed on the order of the Mad King for suggesting that Tywin Lannister, then Hand of the King and whom Ser Ilyn served as Captain of the Guards, was the true ruler of Westeros rather than King Aerys II. He served Robert Baratheon as "the King's Justice" - the royal executioner.
Sansa Stark encounters Ilyn while out walking as they are traveling with the king's procession back to King's Landing, and is startled by his appearance. Sansa apologizes to Ilyn for possibly offending him, but receives no reply. Sandor Clegane informs her that Ilyn had his tongue removed years ago by the Mad King and now serves as the king's executioner.
Ilyn is later ordered to kill Sansa's direwolf Lady by Queen Cersei as revenge for another wolf attacking Joffrey Baratheon. Eddard Stark insists on carrying out the sentence himself instead. Following the death of King Robert I in a hunting accident Ilyn serves King Joffrey I.
Eddard is led before the people outside the Great Sept of Baelor and admits treason. Joffrey tells the crowd that both his Lady Sansa and his mother Cersei have asked that Ned be sent to Castle Black as a punishment. To the surprise of all, however, Joffrey suddenly proclaims that they have the soft hearts of women, and promises that, during his reign, treason will be punished: he orders Ilyn to bring him Ned's head. Ilyn takes House Stark's ancestral sword, Ice, and beheads Ned with one swift stroke.
Marillion is brought before Joffrey because he wrote a song mocking King Robert and Queen Cersei. As a punishment, Joffrey orders Ser Ilyn Payne to cut out Marillion's tongue.
Ilyn Payne's unhesitating obedience in carrying out Joffrey's order to execute Eddard Stark is not without repercussions. When Tyrion Lannister arrives in the capital city, sent by his father Tywin to rein in Joffrey's erratic behavior, he confronts his sister Cersei over her defense that she "tried to stop" Eddard's execution. Tyrion bluntly points out Joffrey wasn't the one holding the sword, yet both Cersei and Ser Ilyn simply went along with the boy-king's orders, when they could have easily just stopped what they were doing.
Ilyn's execution of Ned Stark earns him a place on Arya's death list that she recites each night before she sleeps.
Ser Ilyn continues to serve King Joffrey as the War of the Five Kings ravages Westeros. During the Battle of the Blackwater, Ser Ilyn accompanies Queen Cersei to Maegor's Holdfast. Though the Queen claims Ser Ilyn is present to protect them and to deal with potential betrayers, she later admits to Sansa that Ilyn's true purpose is to execute her and Sansa, and possibly the rest of the ladies who have taken refuge in the holdfast, should Stannis Baratheon take the city.
When Tyrion Lannister confronts his father on the night before his scheduled execution, Tywin assures his son he won't let Ilyn Payne take his head.
Ser Ilyn silently carries out the king's orders of execution, and is therefore known as "The King's Justice". Mad King Aerys had Ilyn's tongue torn out when he heard a rumor that Ser Ilyn had claimed Tywin Lannister held more power than the king. At King Joffrey's command, Ilyn executed Ned Stark.