Dol Guldur
Later Thráin II, the last bearer of one of the seven Rings of Power given to the Dwarves, was captured by Sauron's forces and was kept at Dol Guldur, where he yielded his ring to Sauron under torture. Gandalf went again to Dol Guldur to investigate his suspicions of the power that was rising, learned that this was indeed Sauron, and found Thráin in the dungeons close to death. Thráin had forgotten his own name, but gave Gandalf a map and key to the Lonely Mountain for Thráin's son, Thorin Oakenshield.