When Eärendur, the last king of Arnor died in the year 861 of the Third Age, the country was split into three separate kingdoms: Arthedain, Cardolan and Rhudar. Arthedain, which contained the former capital city of Annúminas was given into the rule of Amlaith, the eldest of Eärendur's three sons. It was through this kingdom that the line of Isildur was maintained, as the line of Arthedain's kings outlasted those of Rhudar and Cardolan.
The borders of Arthedain extended from the Lhun and the Brandywine, and as far east as Weathertop and contained the two palantiri belonging to the Men of the North. One of the Palantiri was located at Amon Sûl. The capital city of Arthedain was at Fornost.
In the year 1349 of the Third Age, after the end of the royal lines
in Rhudar and Cardolan, Argleb I claimed kingship over all of the lands
that originally made up the realm of Arnor. However, that claim was contested,
leading to war, which led to the end of all three regions as formal kingdoms.
After this, the royal family of Arthedain continued in secret as the Chieftains
of the Rangers of the North.