House Thenn is the newest house in the Seven Kingdoms, created in 300 AC when Jon Snow, Lord Commander of the Night's Watch, arranges the marriage of Sigorn, Magnar of Thenn, to Lady Alys Karstark at Castle Black. Jon's purpose is as much rescue as alliance: Alys had fled her cousin Cregan Karstark, who sought to force her into marriage and claim Karhold for himself after already losing, perhaps killing, two wives. Melisandre performs the wedding, and the Night's Watch stewards improvise arms on the spot for the bride's cloak, a bronze disk to honor the Thenn people ringed with red flames for the devotees of R'hllor, echoing the sunburst of House Karstark.
Sigorn keeps his wildling title of Magnar even as a newly sworn lord of the Seven Kingdoms, commanding two hundred warriors equipped, unlike most free folk, with bronze armor and weapons. Should Alys's imprisoned brother Harrion Karstark die without an heir, House Thenn stands to inherit Karhold itself, and by Westerosi custom Sigorn's line may yet be known as Karstark.