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. Dale, a town of Men built between the two southern spurs of Erebor, grew in harmony with the dwarves. 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 numerous and prosperous, well known for the making of matchless weapons and armour.