Battle of the Plains
Prior to the Battle
Twenty one years prior to the battle, the Great Plague had killed thousands of people from east to west. The heaviest of these being in the kingdoms of Rhovanion, which lost half its people, and Gondor who lost many thousands as well. They were slow to recover; but their weakness was not tested for a long time. No doubt the Easterlings had been equally afflicted, so that the enemies of Gondor came mainly from the south or from the sea. But when the invasions of the Wainriders began and involved Gondor in wars that lasted for almost a hundred years, the Northmen bore the brunt of the first assaults.
Though we know little of what happened we do know that King Narmacil II took a great army north to the plains south of Mirkwood, and gathered all he could of the scattered remnants of the Northmen; but he was defeated, and he himself was slain in the battle as well as Marhari leader of the Northemen, who died in the rearguard. The remnant of his army retreated over Dagorlad into Ithilien.
Aftermath and Repercussions
The battle was a disaster for Gondor. It lost it's king, and the lands east of the river Anduin were abandoned, save Ithilien. Narmacil was succeeded by his son Calimehtar. As for the Northmen, a few fled over the river Celduin and merged with the People of Dale (with whom they were akin), some forsook refuge in Gondor, and others were gathering under the leadership of Marhwini son of Marhari. They headed north and eventually settled east of Anduin in the Vales of the Anduin. They were joined by many fugitives who came through Mirkwood. This was the birth of the Éothéod, though no news of this reached Gondor for many years. Most of the Northmen were reduced to servitude, and all their former lands were occupied by the Wainriders.