After many years living peacefully in alliance with Gondor, the Northmen found their lands invaded by a mysterious eastern people known as the Wainriders. Together, the Gondorians and Northmen formed an army to meet the Wainriders, but the resulting Battle of the Plains proved disastrous. King Narmacil II of Gondor was killed in the battle, and so was Marhari of the Northmen. Marhari's son Marhwini fought in the rearguard of the army, and his cavalry was said to have saved many of Gondor's soldiers, but in the end, the Wainriders had the victory.
After the battle, many of the Northmen remained in their old lands under the subjugation of the Wainriders. Others fled north, with some mingling with the people of Dale. Marhwini led another group between the eastern bank of Anduin and the western edge of Mirkwood, settling at last in the northern Vales of Anduin. This was the origin of the Éothéod, the people from whom the Rohirrim would eventually arise.