Karlon Stark is an unseen character in Game of Thrones. He is dead by the time in which the show takes place. He was a younger son of a Lord of Winterfell who was awarded lands on the eastern region of the North after putting down a rebellion of House Bolton of Dreadfort.
Over time, Karlon's seat, known as Karlon's Hold, became known as Karhold. Karlon's descendants, the Karlon's Hold Starks became known as Karstarks.[1][2]
In the books
In the A Song of Ice and Fire books Karlon Stark's story is the same, except that the books do not specify which vassal house the rebel lord belonged to.