The Realm of Arnor (also referred to as the North Kingdom) was established in the year 3320 by Elendil. It was the first of the Kingdoms in Exile established by the Númenóreans (referred to as the Dúnedain) after the sundering of Beleriand.

At its greatest extent it encompassed almost all of the lands between The Misty Mountains in the East and Ered Luin (the Blue Mountains) in the West.

Its capital was originally Annúminas on the shores of Lake Evendim, but by TA861 the city of Fornost on the North Downs had become its largest city and capital.

In that year it was split into the kingdoms of Arthedain, Cardolan, and Rhudaur by the sons of King Eärendur, after his death.

In TA1300, the Kingdom of the Witch-King of Angmar was established to the north-east of Arnor, and made war with Arnor for most of the next 700 years.

By the year TA1409 both Rhudaur and Cardolan were overrun by the forces of the Witch-King but the remaining force of the Dúnedain fought on.

In the year 1975 Arthedain was finally overrun and King Arvedui fled north and perished in the Ice Bay of Forochel. This was considered the final end of the Kingdom of Arnor, and the descendants of Arvedui became known as Chieftains rather than Kings.

By the time of the War of the Ring, the only parts of Arnor that were still inhabited were The Shire, Rivendell and a small area centered around Bree.
Encyclopedia entry originally written by PotbellyHairyfoot