Fornost Erain (Sindarin 'Northern-fortress of the Kings' from for(n) (north) + ost (fortress); "Norbury of the Kings" in Westron) was a city of Eriador in the north of Middle-earth. It was located at the south end of the North Downs, about 100 Númenórean miles north of Bree; after Fornost Erain was abandoned, the site became known as Deadman's Dike, visited only by Rangers. At the time when The Lord of The Rings is set, Fornost had been abandoned for "nearly a thousand years, and even the ruins of Kings' Norbury were covered with grass".
It is not known when Fornost was founded or when the kings of Arnor moved there from Annúminas, but it is known that the kings moved to Fornost some time before T.A. 861, when King Eärendur died, and Arnor was divided into three kingdoms with Fornost the capital of the greatest kingdom, Arthedain.
In T.A. 1974, Arthedain was overrun by the forces of Angmar, they captured Fornost, and King Arvedui fled into the northern wastes and was lost in the Ice Bay of Forochel. In the following year, a fleet from Gondor led by Eärnur landed at Mithlond, fought the Witch-king of Angmar in the plains west of Fornost, and defeated him and his armies, although the Witch-king himself escaped.
After King Elessar reunited the the kingdoms of Gondor and Arnor, it seems he rebuilt the city, and made it the capitol of Arnor once again.