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 a Lord of Winterfell 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 Karlon's Hold, became known as Karhold. Karlon's descendants, the Karlon's Hold Starks became known as Karstarks.

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.