Thráin II was the son of Thror and the father of Thorin Oakenshield. Thráin II fled the city of Dale and the Lonely Mountain with his father and son when it was attacked and taken over by the dragon Smaug in the year T.A. 2770. After wandering in the wilderness, Thráin joined his father and son in attempting to re-settle Moria. After his father was killed by the goblin Azog, Thráin helped his people prepare to avenge his father’s death and rid Moria of the goblins. His efforts culminated in the Battle of Nanduhirion before the East-gate of Moria in T.A. 2799 where his cousin Dain II slew Azog.
Thráin II took up the role of king of his people in exile. In T.A. 2841 he set out for Erebor with the intent to reclaim his kingdom. However, he was pursued by the servants of Sauron, who suspected that Thráin possessed the last of the seven dwarven rings of power. Indeed, Thráin did possess the last of the seven dwarven rings which was wrested from him in T.A. 2845 after being captured, tortured and imprisoned by Sauron in Dol Guldur.
Thráin died imprisoned in Dol Guldur in T.A. 2850, but not before Gandalf entered Dol Guldur, found him and obtained the Map of the Lonely Mountain and the surrounding area, and the Key of Erebor, from Thráin. The map and key would later allow Bilbo to find and enter the secret passage in the Lonely Mountain that would lead him to Smaug’s lair.
Thráin lost an eye atthe Battle of Nanduhirion. “In the morning Thrain stood before them. He had one eye blinded beyond cure and he was halt with leg wound; but he said: “Good! We have the victory. Khazad-dum is ours!” But they (the dwarves) answered: “Durin’s Heir you may be, but even with one eye you should see clearer. We fought this war for vengeance, and vengeance we have taken. But it is not sweet. If this is victory, then our hands are too small to hold it.” (From the appendices to The Lord of the Rings)