Sind.: mountains -- land Probably contracted from "land between mountains".
Area between the MistyMountains and the EredLuin, extending from Forodwaith in the north to the rivers Gwathló and Glanduin. Little known to the Eldar in the FirstAge, it was at first partly under the domain of Melkor, in the SecondAge under the dark shadow of Sauron. At the end of the SecondAge Arnor was founded there. Prosperous at first in the ThirdAge the NorthernKingdom began to fall apart towards the end of the second Millenium. At the end of the ThirdAge except for a few scattered settlements only the Shire, Bree land and Rivendell were populated.
Eriador had once been covered with vast forests, but in the SecondAge much of these had been removed by the Númenóreans. Tar-Aldarion built the great port of Lond Daer at the mouth of the Gwathló to facilitate the shipping of timber. Such was the devastation wrought upon these lands by the Númenóreans that, as Unfinished Tales tells, the "native" folk of Minhiriath (actually descendants of the Folk of Haleth, and so distant kin of the Dunedain) allied themselves with Sauron, and harried the invaders from Westernesse. Many of these were driven off by the Dunedain, seeking refuge in the ErynVorn, where they became a secretive hunter-folk.
Not all folk, however, were hostile to the newcomers: indeed, there were many inhabitants of Eriador who were descended from the Edain and who welcomed the Dunedain as friends and teachers. These dwelt about Lake Evendim, in the North Downs and the Weather Hills, and along the Baranduin--but they were mostly destroyed in the War with Sauron. It is also rumoured that in the northern and eastern regions of Eriador there dwelt a remnant of the Easterlings of Beleriand, perhaps the earliest inhabitants of Eriador in the SeA. Whether they were in origin good or evil is not told: in the Dark Years they, like the Dead Men of Dunharrow, undoubtedly came to worship Sauron (but were possibly subdued by Elendil and Isildur when Arnor was founded). These may have been the forebears of the Hillmen of Rhudaur.
Eriador had also been the location of the Noldorin realm of Eregion, and it was here that the Rings of Power were forged by the Gwaith-i-Mírdain and Sauron (in disguise). Soon after Celebrimbor discovered the treachery of Sauron with the forging of the OneRing, Sauron assailed and laid waste to Eregion. Though his forces were great, he was assisted by many peoples who had been dispossessed by the Númenóreans, and thus had reason to hate the West. With their aid, Sauron's forces were able to overrun Eriador as far the River Lhûn. Elrond, leader of the forces who resisted Sauron, managed to escape with the survivors of the Gwaith-i-Mírdain, and founded Rivendell in SeA 1697, which would remain a beacon of resistance against the Enemy until the WotR. Thus began the Dark Years.
When Sauron's forces were destroyed or driven off by the hosts of Tar-Minastir in 1700, a few folk began to move back into Eriador. Branches of a people akin to the Dunlendings settled in regions close to the Barrow-downs, and in time became the Men of Bree. Nonetheless, by the time the realm of Arnor was founded in 3320, much of Eriador was virtually empty.