The title taken by the dwarf Thráin I who discovered Erebor, the Lonely Mountain and founded a Kingdom there.

Thráin’s son Thorin I removed to the Grey Mountains in the North and although he became King on his father’s death in TA 2190, he did not return to Erebor. For more than 400 years his descendants remained in the Grey Mountains, but in TA 2589, when Dain I and his second son Frór were killed by a cold-drake, Dain’s heir, Thrór brought his family back to Erebor and took up the title again.

The Kingdom prospered but in TA 2770, the dragon Smaug descended on the mountain in flame, killing many and driving Thrór and his kin into the wilderness. With those few who had survived they eventually settled in the Ered Luin beyond Eriador, where Thrór was King in Exile. Eventually, old and homesick, Thrór entrusted to his son Thráin II his elven ring and the map and key of Erebor, and wandered away. He was murdered by Azog the orc, father of Bolg.

Thráin II now King in Exile, eventually set out himself, with the intent of reclaiming Erebor but was captured by the Necromancer, robbed of the ring and consigned to the dungeons of Dol Guldur. There Gandalf found him and was entrusted with the map and key as Thráin breathed his last.

Eventually, Gandalf met Thorin II (Oakenshield) and realising who the dying prisoner had been, gave him the map and key and arranged an expedition to recover the Lonely Mountain. The Quest of Erebor is recounted by Bilbo Baggins in The Hobbit.

After the battle of Five Armies, Thorin Oakenshield was briefly King under the Mountain but died of his wounds shortly thereafter. He was entombed in the heart of the mountain and his heir and second-cousin Dain II (Ironfoot) took up the title and restored the Kingdom.

NOTE: Additional information from Cirdaneth.
Encyclopedia entry originally written by Daughter_of_Kings