Cregan Karstark is the eldest son of Arnolf Karstark. He has a younger brother, Arthor Karstark. Cregan has been married twice before, and Alys Karstark suspects he killed both his wives. Though he was a cousin, the children of his first cousin Lord Rickard Karstark called him uncle.
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 finds out about her "uncles'" plans and rides to the Wall to seek hope from (she thinks) Ned Stark's last surviving son, Jon Snow. She thinks once she has borne Cregan a child he will kill her. Cregan pursues her with with four mounted men-at-arms, a dog trainer, and a pack of hounds. Alys arrives at the Wall and reveals the Karstark plot. The next day Cregan arrives but is met by Jon Snow and some of his rangers at Mole Town before he can reach the Wall and claim guest right. One of the men loses a crossbow bolt and dies for it, the rest of Cregan's men surrender. Cregan and the four remaining men are imprisoned in ice cells.
Jon sends a letter to Stannis reporting the Karstark treachery and arranges Alys' marriage to Sigorn, the new Magnar of Thenn, as part of his plan to integrate the Wildlings into the North. Cregan complains to Jon of guest right being violated, but Jon tells him they aren't guests, having come here to take Cregan's niece back to Karhold and force her into marriage. Cregan claims she was promised to him but Jon says Arnolf is only a castellan and has no right to make marriage pacts. Cregan accuses Jon of being a kinslayer, like his half-brother Robb, as Starks and Karstarks are kin. Jon tells Cregan he can either take the Black, or be hanged when Stannis returns. Cregan becomes more unstable over time, throwing his faeces at the men. Due to snow burying the doors to the Ice cells, the men are moved out and imprisoned elsewhere.
Stannis receives' the warning and arrests Arnolf and the Karstarks with him, telling them they will die.