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East Lórien was a realm of the Elves founded in southern Eryn Lasgalen (formerly Mirkwood) in the wake of the War of the Ring. It was centred around Amon Lanc, the former site of the Necromancer's fortress of Dol Guldur. Many of its people were Galadhrim formerly of Lothlórien who chose to abandon the Golden Wood and resettle in the cleansed eastern forest. Celeborn was its first and last known ruler.
The borders of East Lórien included all the forest from its southern eaves north to the Narrows. At its greatest extent, the realm stretched two hundred miles east to west and over a hundred miles north to south. The forest north of the Narrows was populated by Beornings and Woodmen.
In the Second Age, Oropher established the capital of his Woodland Realm upon the hill of Amon Lanc, but the Silvan Elves abandoned this settlement for unclear reasons. The ruin lay empty until roughly T.A. 1000, at which time the Necromancer claimed it as his abode and the place became known as Dol Guldur.
During the War of the Ring, Galadriel, Celeborn, and the Elves of Lórien tore down the stronghold of Dol Guldur. After the victory, on the day of the New Year of the Elves (6 April T.A. 3019), Celeborn met with the Elvenking Thranduil of the Woodland Realm in the midst of the forest. The two agreed to rename the cleansed Mirkwood to Eryn Lasgalen and divide it between themselves and the Beornings and Woodmen. Thranduil claimed the northern third of the forest, Celeborn claimed the south, and the Beornings and Woodmen were granted the middle.
Celeborn ruled his new realm into the Fourth Age, though he did not remain there long. After a span of only "a few years," he retired to Rivendell to stay with his grandsons, the sons of Elrond. It is unknown if East Lórien continued as a realm after the departure of its founder, and if so, who may have succeeded him – if anyone. All that is known is that "in the Greenwood the Silvan Elves remained untroubled" for some time thereafter.