Heir of Isildur
Heir of Isildur was the title of thirty-nine lords of the Dúnedain descended in right line from Isildur the son of Elendil, from his son Valandil to Aragorn Elessar, who reunited the Kingdoms of the Dúnedain in Middle-earth.
Isildur was the eldest son of Elendil, and so by right he and his heirs should have inherited the title of High King and ruled the Two Kingdoms of Arnor and Gondor. Soon after the defeat of Sauron at the end of the Second Age, though, Isildur and his three elder sons all perished in the Disaster of the Gladden Fields.
Isildur's only remaining Heir was Valandil, kept safe in Rivendell but only eleven years old at that time, and unable to take up his rule. So, Isildur's nephew Meneldil took control of the kingdom of Gondor, and the Two Kingdoms split from one another. In the south, Gondor remained strong under the rule of Meneldil's descendants, but the North-kingdom was not so fortunate. It broke into three separate kingdoms, and was slowly overcome by dark forces of the neighbouring land of Angmar. Arvedui, Isildur's Heir through twenty-three generations, was the last King to rule in the north; he attempted to reclaim the High Kingship, but failed, and the last remnant of royal Arnor was overrun by its enemies.
Now the line of Isildur in the north became forgotten by nearly all. Though their royal cities were wasted, and their people dwindled, the line of Isildur's Heirs continued for another sixteen generations under the title 'Chieftain of the Dúnedain'. The last Chieftain was Aragorn II of the War of the Ring, who famously came to Minas Tirith and reclaimed the High Kingship of all the western lands as the Reunited Kingdom.