Rhûn, also known as The East and Eastlands in the Westron tongue, is a currently unaccessible region in the far eastern part of Middle-earth. It is home to the Easterlings.
"Rhûn" is a name used for all lands lying east of Rhovanion, around and beyond the inland Sea of Rhûn, from where many attacks on Gondor and its allies came during the Third Age. We know almost nothing of the lands beyond the great Sea of Rhûn that stood on its borders with the western lands. Even Gandalf has never explored there, and although Aragorn visited once, we have no report of his activities.
Far beyond the Sea of Rhûn was another inland sea, the Sea of Helcar, and beyond that a range of red mountains known as the Orocarni (Red Mountains). Somewhere in the lost east, too, lay Cuiviénen and Hildórien, where Elves and Men first awoke: all the Children of Ilúvatar could trace their ancestries back to the eastward regions of Middle-earth.
Rhûn was the domain of the Easterlings, Men of Darkness who were ready to follow both the Dark Lords and fought as their allies in war. These lands, too, were inhabited by lost Elves, Avari and Úmanyar, and by four of the seven clans of the Dwarves. Mumakil or Oliphaunts, of Far Harad, also probably lived in this area, but it is unknown.
During the Third Age, Rhûn was visited by three Wizards; Saruman, Alatar and Pallando, and though Saruman returned into the west, the two Blue Wizards remained. Sauron himself journeyed into the eastward lands, in hiding from the White Council during the centuries known in the west as the Watchful Peace.
Dorwinion lay in Rhûn, and the Easterlings of the Balchoth and Wainriders lived there.
Gondor conquered Rhûn twice: under the Kings Rómendacil I and Rómendacil II, but the Númenóreans never had full control over it.