Karlon Stark

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 the King in the North who was awarded lands on the eastern region of the North after putting down a rebellion of House Bolton of the Dreadfort.

Over time, Karlon's seat, known as Karl's Hold, became known as Karhold. Karlon's descendants, the Karl's Hold Starks became known as Karstarks.

In the books
In A Song of Ice and Fire, Karlon Stark's story is the same, except that the books do not specify which vassal house the rebel lord belonged to.