The Men of Rhûn. Frequently attacked Gondor, probably inspired by Sauron.
Should not be confused with the Easterlings who entered Beleriand in the First Age, and fought for both the Elves and Morgoth. Many of these Easterlings did not survive the cataclysms at the end of the First Age, but some of those who did might have settled in Eriador and become the Hillmen of Rhudaur.
The term "Easterling" later applied to the various peoples who assailed Gondor from the East. Sometimes this involved the migration of an entire people, such as that of the Balchoth who occupied Rhovanion in the latter Third Age, and who later attempted to take the province of Calenardhon in Gondor. Five hundred years earlier, an Easterling people known as the Wainriders had also occupied Rhovanion, and their continuous incursions into Gondor were sapping the resources of the Dunedain. When the Northmen of Rhovanion revolted against the occupiers, Calimehtar of Gondor seized the opportunity to force the issue in open battle on the plain of Dagorlad, and the Wainriders were driven off or destroyed.
Towards the end of the Third Age, Easterling incursions were more strategic and co-ordinated, in alliance with Mordor and the Harad. Moreover, under Sauron's leadership they no longer concentrated their assaults solely upon Gondor, but also upon her allies and upon other realms unfriendly to Sauron. While many Easterlings fought for Sauron at the Battle of the Pelennor, they also beseiged Dale and Erebor: had they defeated these realms, they clearly would have moved onto Rivendell and Eriador.