In the literary works of J. R. R. Tolkien, the House of Isildur was the Royal House of Arnor, Arthedain, Cardolan, Rhudaur and, much later, Gondor The house was descended from Isildur, elder son and heir of Elendil the Faithful. Although the North-Kingdom was ended in the year 1975 of the Third Age, the House of Isildur was maintained as the Chieftains of the Dúnedain, the wandering folk that the Dúnedain of Arnor became, until the time of Aragorn II and the War of the Ring, after which, Aragorn claimed the Crown of Gondor as the direct descendant of Elendil. After Aragorn, the House of Isildur became the House of Telcontar and ruled the Reunited Kingdom for generations.
It was the pride and wonder of the Northern Line that, though their power departed and their people dwindled, through all the many generations the succession was unbroken from father to son. ... But in Aragorn the dignity of the kings of old was restored
However, in Cardolan, the Line of Isildur died out and was replaced by local noblemen. Cardolan recognised therefore the overlordship of Arthedain, though Rhudaur gave its loyalty to the Witch-king and Angmar which led to long hostilities, although most of the Dúnedain of Rhudaur left for Cardolan and Arthedain.