In J. R. R. Tolkien's legendarium, the Lonely Mountain is a mountain northeast of Mirkwood. It is the location of the Dwarves' Kingdom under the Mountain and the town of Dale lies in a vale on its southern slopes. In The Lord of the Rings, the mountain is called by the Sindarin name Erebor. The Lonely Mountain is the destination of the protagonists, including the titular Hobbit Bilbo Baggins in The Hobbit, and is the scene of the novel's climax.
The mountain is the goal of Bilbo's psychological quest in The Hobbit; scholars have noted that it and The Lord of the Rings are both structured as quests to a distant mountain, but that the quests have very different motivations. Further, the mountain is a symbol of adventure in The Hobbit, and of Bilbo's maturation as an individual, while to the Dwarves, it stands for the gain of beauty in return for loss of life.
Fictional mountain
Erebor stands hundreds of miles from the nearest mountain range. Tolkien's rendering of Thrór's map in The Hobbit shows it with six ridges stretching out from a central peak that was snowcapped well into spring. The whole mountain is some ten miles in diameter; it contains an immense wealth of gold and jewels.
Origins
Erebor becomes the home of the Folk of Durin, a clan of Dwarves known as the Longbeards, after they are driven from their ancestral home of Khazad-dûm. In the latter days of the Third Age, this Kingdom under the Mountain holds one of the largest dwarvish treasure hoards in Middle-earth. Dale, a town of Men built between the two southern spurs of Erebor, grew in harmony with the dwarves. The Kingdom under the Mountain is founded by Thráin I the Old, who had discovered the Arkenstone there. His son, Thorin I, leaves 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 plunder their hoards, the Longbeards, led now by Thrór, a descendant of Thorin, return to Erebor to take up the title King under the Mountain. Under Thrór's reign, Erebor becomes a great stronghold where the dwarves are numerous and prosperous.
Erebor in The Hobbit
In the Third Age, while the young Thorin II Oakenshield is out hunting, the dragon Smaug flies south from the Grey Mountains, kills all the dwarves he could find, and destroys the town of Dale. Smaug takes over the mountain, using the dwarves' hoard as a bed. King Thrór, his son Thráin II, and several companions escape death by a secret door. While Thrór and Thráin later perish, Thorin lives in exile in the Ered Luin, far to the west. On a journey, he meets the wizard Gandalf. Together they form a plan to reclaim the mountain. Gandalf insists that burglary is the best approach and recommends the hobbit Bilbo Baggins.
Bilbo, Thorin, and Thorin's company of twelve other Dwarves travel to the Lonely Mountain to regain the treasure. They plan to use the secret door, whose key and map Gandalf had obtained 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 are in the sky together, the day's last sunlight falls on the door and exposes its keyhole. The Hobbit enters the mountain and steals a golden cup.
Smaug, enraged by the theft, emerges from the mountain and flies south to destroy Lake-town, which he suspects is the source of the "thieves". During this attack Smaug is killed by Bard the Bowman; Thorin claims the mountain on learning of Smaug's demise. However, the Men of Esgaroth, supported by Thranduil and the Elves of Mirkwood, march in force to the mountain to demand a part of the dragon's hoard as recompense for the destruction. Thorin, mad with greed, refuses all claims and sends word to his second cousin Dáin II Ironfoot, chief of the Dwarves of the Iron Hills, who bring reinforcements. Before battle can begin, an army of Orcs and Wargs descends on Erebor. Dwarves, Elves, and Men join ranks against them, leading to the Battle of Five Armies. Thorin's nephews Fíli and Kíli are killed, and Thorin is mortally injured; he dies shortly afterwards. The title of King under the Mountain passes to Dáin.