Thorin’s father, Thráin II, was captured by Sauron and imprisoned in Dol Guldur, where he died.
Thráin II, son of Thrór and father of Thorin Oakenshield, was the King of Durin’s Folk during a tumultuous period in the history of the Dwarves. After the dragon Smaug drove the Dwarves out of the Lonely Mountain (Erebor), Thráin and his people lived in exile. Years later, Thráin set out to return to Erebor but was waylaid by misfortune. He was captured by the forces of Sauron, who at that time was disguised as the Necromancer and occupied the fortress of Dol Guldur in Mirkwood.
Thráin was imprisoned in the dungeons of Dol Guldur, where he endured torture and hardship. Sauron sought to learn from him the whereabouts of the last of the Seven Dwarven Rings of Power, which Thráin possessed. However, the ring had already been taken from him by Sauron’s agents before his capture. During his imprisonment, Thráin was driven to madness and despair, and he eventually died in captivity without ever revealing any information to Sauron.
Gandalf the Grey later entered Dol Guldur on a reconnaissance mission and discovered Thráin in the dungeons. By then, Thráin was near death and could not remember his own name, but he gave Gandalf a map and key to the Lonely Mountain, which were crucial in Thorin’s quest to reclaim Erebor. Thráin’s death and the loss of his ring marked a tragic chapter in the history of the Dwarves, and his fate weighed heavily on Thorin as he sought to restore his kingdom.