The North Kingdom of the Dúnedain, Arnor was founded in the year
3320 of the Second Age, after the Downfall of Númenor. Its founder
was Elendil, who was the High King of both Arnor and Gondor (founded
by his two sons, Isildur and Anárion). The tenth King of Arnor,
Eärendur, split the realm between his three sons, so it was later
called Arthedain, Cardolan, and Rhudaur. In Cardolan and Rhudaur,
the Dúnedain soon dwindled enough that by 1350 of the Third Age,
it was overtly allied with Angmar, the realm of the Witch-King.
After the destruction of Angmar, the line of Arnor's Kings never
failed, though Arnor was ruined, and the Kings were called Chieftains
instead. It was from this line that the Kings of Gondor eventually
came from too, with the end of the Third Age and Aragorn's crowning.