Arthor Karstark is the second son of Arnolf Karstark. His older brother is Cregan Karstark. Arthor has three sons.
After Rickard Karstark's execution by Robb Stark for murder and treason, Rickard's only surviving son Harrion Karstark becomes Lord of Karhold. However they are a captive of the Lannisters. The heir to Karhold is Rickard's daughter Alys Karstark. However Rickard's uncle Arnolf Karstark has been left as Castellan of Karhold and is de facto lord.
When Stannis Baratheon arrives in the North, he sends out messages to the Northern lords for support. The only "lord" to declare for him is Arnolf Karstark. However Arnolf hopes that when news of this reaches the Lannisters his great-nephew Harrion will be executed. He plots to force Alys to marry Cregan, enabling his branch of the family to take over Karhold. Arnolf is working with the Boltons and plans to betray Stannis.
However Alys rides to the Wall, pursued by Cregan. She seeks assistance from Jon Snow, who imprisons Cregan and his men and marries Alys to Sigorn, the new Magnar of Thenn.
Meanwhile Arnolf and his forces meet with Stannis in a Crofter's village three days from Winterfell. The force includes Arthor and his three sons. While there, Arnolf encourages Stannis to attack Winterfell, intending to betray him, but Stannis delays.
A message arrives from Jon Snow via Tycho Nestoris, informing Stannis of the Karstark treachery. Stannis has Theon chained in the Watchtower and interrogates him. He then invites Arnolf, Arthor, and Arnolf's three grandsons in to break their fast. Arnolf sees Theon and realises his plan has been discovered, while Arthor comments that there are no chairs. Stannis' knights surround the Karstarks and Stannis tells them they can either have a quick death if they confess, or be burnt if they waste his time. One of the grandsons tries to pull out his sword but has his arm cut off, and one of his brothers is wounded in the chest, while Arthor and his father remain still. The Karstarks are then taken into confinement.