An isolated peak east of northern Mirkwood, and west of the Iron Hills, source of the River Running. Settled by dwarves in the Third Age it was the site of the Kingdom under the Mountain and was later known by the Sindarin name of Erebor.

The Lonely Mountain was settled by Thrain I of Durin's Folk in TA 1999 after the dwarves were driven from Khazad-dûm in TA1981 by a Balrog. He named it the Kingdom under the Mountain and found there the Arkenstone, Heart of the Mountain. His son Thorin I sought greater riches in the Ered Mithrin but after his descendant Dain I was slain by a cold-drake in TA 2589, Thror led his people back to the Lonely Mountain arriving the following year.

In TA 2770 the dragon Smaug, descended upon the Lonely Mountain driving the dwarves forth yet again, killing many. Devastating the local area, Smaug assembled a dragon-hoard in a deep chamber under the mountain and there remained unapproachable until Thorin and Company, with the hobbit, Bilbo Baggins, flushed him out in TA 2941 to be slain by Bard the Bowman. There followed the Battle of Five Armies in which Thorin II, Oakenshield, was slain, his successor Dain II, Ironfoot, became King under the Mountain and restored the beauty of the ancient halls. The renewed strength of this stronghold was crucial in the final stages of the War of the Ring.

In the War of the Ring, the Lonely Mountain was attacked by Easterlings under the rule of Sauron, but the combined forces of dwarves and the men of Dale withstood the assault until news came of Sauron's downfall.

In the records of Gondor and the Reunited Kingdom, the Lonely Mountain was referred to as Erebor, the Sindarin form of its name. See that entry for further information.

Note: Additional information from Hathaldir and Cirdaneth.