|Place from J. R. R. Tolkien's legendarium|
Artist's depiction of the Lonely Mountain
|Location||Northeast of Mirkwood|
In J. R. R. Tolkien's Middle-earth legendarium, the Lonely Mountain (Sindarin Erebor) is a mountain in the north of Rhovanion. It is the source of the Celduin River, and the location of the Kingdom Under the Mountain. The town of Dale lies in a vale on its southern slopes.
Internal history 
Origins of the Kingdom Under the Mountain 
Erebor became the home of the Folk of Durin, a clan of Dwarves known as the Longbeards, after they were driven from their ancestral home of Khazad-dûm. In the latter days of the Third Age, this Kingdom Under the Mountain held one of the largest dwarvish treasure hoards in Middle-earth.
The mountain stood hundreds of miles from the nearest mountain range. Tolkien's map shown in The Hobbit shows the mountain with six ridges stretching out from a central peak that was snowcapped well into spring. The whole mountain was perhaps ten miles in diameter, with Dale, a town of Men, built between the two southern spurs.
The Kingdom Under the Mountain was founded by Thráin I the Old, who discovered the Arkenstone there. His son, Thorin I, left the mountain with much of the Folk of Durin to live in the Ered Mithrin (Grey Mountains) on account of the great riches to be found in that range. After dragons plundered their hoards, the Longbeards, led now by Thrór, a descendant of Thorin, returned to Erebor to take up the title King under the Mountain. Under Thrór's reign, Erebor became a great stronghold where the dwarves became a numerous and prosperous people. The Dwarves of Erebor were at that time well known for making of matchless weapons and armour, and there was great demand for their work by the surrounding peoples. The Longbeards amassed a large treasure hoard during this time.
Erebor in The Hobbit 
In 2770 T.A., while the young Thorin II Oakenshield was out hunting, the dragon Smaug flew south from the Grey Mountains, killed all the dwarves he could find, and destroyed the town of Dale. Smaug then took over the mountain, using the dwarves' hoard as a bed. King Thrór, his son Thráin II, and several companions escaped death by a secret door. Although Thrór and Thráin later perished, Thorin lived in exile in the Ered Luin until, by chance, he met the wizard Gandalf. Together they formed a plan to reclaim the mountain. Gandalf insisted that burglary was the best approach and recommended the hobbit Bilbo Baggins, whom he represented to be a professional thief.
Thus Bilbo, Thorin, and Thorin's company of twelve other Dwarves travelled to the Lonely Mountain to regain the treasure. They planned to use the secret door, whose key Gandalf had managed to obtain from Thráin, whom he had found at the point of death in the pits of Dol Guldur. On Durin's Day, when the setting sun and the last moon of autumn were in the sky together, the day's last sunlight would fall on the door and expose its keyhole so that it could be unlocked. By a fortunate coincidence, this happened soon after Bilbo and the dwarves arrived, and the hobbit was able to enter the mountain and steal a golden cup.
Smaug, enraged by the theft, emerged from the mountain and destroyed Lake-town, which he thought to be the source of the "thieves". During this attack Smaug was slain by Bard the Bowman, and Thorin claimed the mountain. However, the Men of Esgaroth, supported by Thranduil and the Elves of Mirkwood, demanded a part of the dragon's hoard as recompense for the destruction. Thorin, mad with greed, refused all claims and sent word to Dáin II Ironfoot, chief of the Dwarves of the Iron Hills, who brought reinforcements to the aid of Thorin and Company. Before the battle began, however, an army of orcs and wargs descended on Erebor. Dwarves, elves, and men joined ranks against them, which led to the Battle of Five Armies. During the battle, Thorin's nephews Fíli and Kíli were slain, and Thorin himself was mortally injured and died shortly afterwards. The title of King under the Mountain passed to Dáin.
Erebor in The Lord of the Rings 
With the restoration of the Kingdom under the Mountain the area became prosperous again. Dale was rebuilt under Bard's leadership, and dwarves and men reforged their friendship.
During the War of the Ring, Dale was overrun by Sauron's northern army, and many dwarves and men took refuge in Erebor, which was promptly surrounded. Dáin II was killed before the gates of Erebor defending the body of his fallen ally King Brand of Dale. Dáin was succeeded by his son Thorin III Stonehelm, who along with King Bard II withstood the siege and later routed Sauron's forces. In "The Quest of Erebor", Gandalf relates that although Sauron's main thrust was directed against Gondor, the restoration of the Kingdom Under the Mountain provided a bulwark against Sauron's northern allies, who otherwise could have done great harm in Arnor and the Shire.
The Lonely Mountain: Lair of Smaug the Dragon is a board game produced in 1985 by Iron Crown Enterprises, designed by Coleman Charlton which features groups of adventurers, either Dwarves, Elves, Orcs or Men entering Smaug's Lair to capture his treasure before he awakens.
"Erebor", specifically the southern spurs of the Mountain and Dale, is a playable map in The Lord of the Rings: The Battle for Middle-earth II. It has three gates, including the one Tolkien described and two which cannot be closed, to allow those playing as invading forces to easily enter the stronghold.
The International Astronomical Union names all mountains on Saturn's moon Titan after mountains in Tolkien's work. In 2012, they named a Titanian mountain "Erebor Mons" after the Lonely Mountain.
See also 
- Fonstad, Karen Wynn. The Atlas of Middle-earth (Revised Edition) . Houghton Mifflin Company. New York. 1991. pp. 110-1
- Newsboard, Fellowship Follows, White Dwarf (magazine) #57, September, 1984 p45
- International Astronomical Union. "Categories for Naming Features on Planets and Satellites". Gazetteer of Planetary Nomenclature. Accessed Nov 14, 2012.
- International Astronomical Union. "Erebor Mons". Gazetteer of Planetary Nomenclature. Accessed Nov 14, 2012.
- Harper, Amelia (2006). "Lonely Mountain (Erebor)". In Drout, Michael D. C.. J. R. R. Tolkien Encyclopedia: Scholarship and Critical Assessment. Routledge. pp. 384–385. ISBN 0-415-96942-5.