J.R.R. Tolkien> The Lord of the RIngs
Erebor is the mountain east of Mirkwood and west of the Iron Hills that Thrain I mined out and settled after the fall of Khazad-dûm. Erebor grew in glory unprotested for nearly 200 years until the worm Smaug plundered it in the year 2770 of the Third Age. The dragon held his keep for as long as the dwarves did; in 2941 he was invaded by Thorin and Company and later slain by Bard. This is when Dáin II took the title of King Under the Mountain and re-established the dwarf-kingdom of old. After the War of the Ring, Erebor allied with the Reunited Kingdom, but chose to remain independant.
In the Hobbit it is refered to as the Lonely Mountain.