In J.R.R. Tolkien's The Hobbit, Thorin Oakenshield leads the expedition to reclaim Erebor.
Thorin is the son of Thráin II and the grandson of Thrór, the former King under the Mountain. After Smaug drives the Dwarves from Erebor, Thorin grows up in exile and eventually organizes the journey back to the Lonely Mountain. He hires Bilbo Baggins as the company's burglar and seeks Gandalf's assistance.
The expedition succeeds in entering Erebor, but reclaiming the treasure creates a new conflict. Thorin becomes increasingly possessive of the hoard, especially the Arkenstone, and refuses to share the mountain's wealth with the claimants gathered outside. His pride is challenged by Bilbo, who secretly gives the Arkenstone to Bard and the Elvenking.
Thorin later fights in the Battle of the Five Armies and dies from his wounds. Dáin Ironfoot succeeds him as King under the Mountain. Thorin's epithet refers to his use of an oak branch as a shield during an earlier battle.