King under the Mountain

The term King under the Mountain was used in a good deal of Celtic mythology (and later fairy tales) to refer to the ruler of the supernatural creatures particular to a given location, particularly the sidhe.

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In J. R. R. Tolkien's fantasy universe of Middle-earth, the King under the Mountain is the title of the Dwarf ruler who resides at the Lonely Mountain.

In The Hobbit, the dragon Smaug held this title. Thorin was in line to be the King under the Mountain.