The region between the Blue Mountains and the Misty Mountains has been called Eriador by the elves throughout the First, Second and Third Ages.

During the First Age its population was mostly loyal to Morgoth and he used it for attacks on the elves of Beleriand.

In the Second Age, Eriador was inhabited mostly by the ancestors of the Dunlendings, who were allied with the Dark Lord Sauron.

In the year 3320 of the Second Age Elendil, King of the Dunedain (Númenorians in exile) founded the Kingdom of Arnor and from then until the year 1300 of the Third Age all of Eriador became part of this kingdom.

In that year the Witch-Kingdom of Angmar was was formed in the North by the Lord of the Nazgul and fought wars with the Dúnedain leading to the death of the last king in the year 1974. In 1975 the forces of the Witch-king were defeated in the Battle of Fornost and the Kingdom of Angmar was destroyed.

The wars with the Witch-King decimated Eriador and most of the surviving population fled. At the time of the War of the Ring the only areas of Eriador that were still populated were The Shire, Bree and Rivendell.
Encyclopedia entry originally written by PotbellyHairyfoot