Great Hall of Thráin
The Great Hall of Thráin was a vast rocky chamber beneath the deepest part of the Lonely Mountain of Erebor, named for the founder of the Kingdom, Thráin I. It held the Arkenstone, which Thráin himself had found beneath the Mountain, and the stone spilled light into the Hall (at least, if the songs of the Dwarves are to be believed).
After a time, the Kings of Durin's line abandoned Erebor, and for a time the Great Hall was without the light of the Arkenstone. After nearly four centuries, Thráin's descendant Thrór returned there, and replaced the brilliant stone in the Hall. For nearly two hundred years more the Dwarves dwelt undisturbed beneath Erebor, until the coming of the dragon Smaug. Thrór and his family were able to escape through a secret passage that led from the Great Hall, but Smaug plundered their treasures and gathered them together, lying on a bed of gold in the Great Hall of the Kings under the Mountain.
Many years later, Bilbo Baggins crept back down the same passage to find Smaug sleeping on his golden hoard. Bilbo failed to recognise the chamber as a Hall of Kings: he described it as no more than 'the great bottommost cellar or dungeon-hall of the ancient dwarves'. Through a series of extraordinary events, Smaug was eventually defeated and the line of the Kings under the Mountain was restored. Dáin II Ironfoot took up the Kingship of Erebor, but he did not return the Arkenstone to its place in the Great Hall. Instead, it was buried on the breast of his cousin Thorin Oakenshield, who had died in the battle to regain the Kingdom.