House Karstark after Rickard Karstark's death
With Rickard's son Harrion still held captive in Maidenpool, later under command of Lord Randyll Tarly, Arnolf Karstark rules Karhold as the castellan. He is the only one of the Northern lords to declare for Stannis Baratheon when they arrive in the North but he's truly loyal to the Boltons. He first appears in the series when he is seen by Theon Greyjoy feasting with Ramsay Bolton at the Dreadfort. Arnolf plans to lure Stannis into attacking the Dreadfort, then turn on him when he does so. However this plan is foiled when on Jon Snow's advice Stannis decides to attack Deepwood Motte instead, defeating the last centre of Ironborn power in the North.
Arnolf and his elder son Cregan conspire to have Harrion Karstark killed by the Lannisters, to enable Arnolf to take control of Karhold. He plans to marry Alys Karstark to Cregan, and the girl fears for her brothers' life as well as her own, since she thinks Cregan will kill her after she gives him a son. Alys flees to the Wall, chased by Cregan and Karstark soldiers. She arrives at Castle Black and warns Jon Snow of Roose Bolton and Arnolf Karstark's plot to betray Stannis at Winterfell. Jon Snow let Alys stay in Castle Black under his protection, and writes a letter to Stannis to warn him about Arnolf. Cregan arrives at the Wall with his men and they are all imprisoned. Jon offers Cregan a choice to take the black or wait until Stannis returns at Castle Black and hangs him. Alys Karstark is married to Sigorn, the new Magnar of Thenn and son of Styr. The wedding was attended by Jon, Queen Selyse, Princess Shireen, Ser Axell Florent, Val, Torghen Flint, Brandon Norrey, men of the Night's Watch, House Baratheon of Dragonstone and Mole's Town, the wildlings, and Wun Wun the giant. The marriage is conducted by Lady Melisandre. Sigorn becomes the founder of a new noble house of the North: House Thenn.
Arnolf's forces, including himself, meet Stannis at a crofter's village three days from Winterfell. He urges an attack on Winterfell so he will have an opportunity to betray Stannis, but is unable to convince Stannis' men, who say the plan will probably fail.
The representative of the Iron Bank of Braavos, Tycho Nestoris bring Jon Snow's message to Stannis Baratheon. He finds his army camped at the crofters' village, starving in the snow in the desperate march on Winterfell against the Bolton. He arrives to King Stannis with Ramsay Bolton's wife "Arya Stark" (Jeyne Poole), Theon Greyjoy, and other Ironborn captives from Deepwood Motte. Stannis has Theon chained and interrogates him. After finding out from Maester Tybald that Arnolf sent a map to Roose Bolton showing Stannis' location, Arnolf, his younger son Arthor, and three grandsons of Arnolf come in, having been invited to break their fast with Stannis. Stannis' knights surround them, injuring two of Arnolf's grandsons. Stannis promises that all five of them will die but whether quickly or burned alive will depend on their cooperation. Meanwhile, the Karstark men are confined to the hall, though Stannis believes they did not know of Arnolf's scheme, or it could not have been kept secret.
- With his hunchback, his plotting to get his (great-)nephew killed, and marry his (great-)niece to her "uncle" (first cousin once removed) and the Karstark sigil being a sun and its words "the Sun in Winter", Arnolf has similarities to the theatrical Richard III.