Thrain IIThráin II - Son of Thrór and father of Thorin II (Oakenshield).  He became king of the Dwarves when his father was slain by Orcs in Moria (which initiated the War of the Dwarves and the Orcs).  Unfortunately, he met with a tragic fate.

 

"It was therefore perhaps partly by the malice of the Ring that Thráin after some years became restless and discontented.  The lust for gold was ever on his mind.  At last, when he could endure it no longer, he turned his thoughts to Erebor, and resolved to go back there.  He said nothing to Thorin of what was in his heart; but with Balin and Dwalin and a few others, he arose and said farewell and departed.  Little is known of what happened to him afterwards.  It would now seem that as soon as he was abroad with few companions he was hunted by the emissaries of Sauron.  Wolves pursued him, Orcs waylaid him, evil birds shadowed his path, and the more he strove to go north the more misfortunes opposed him.  There came a dark night when he and his companions were wandering in the land beyond Anduin, and they were driven by a black rain to take shelter under the eaves of Mirkwood.  In the morning he was gone from the camp, and his companions called him in vain.  They searched for him many days, until at last giving up hope they departed and came at length back to Thorin.  Only long after was it learned that Thráin had been taken alive and brought to the pits of Dol Guldur.  There he was tormented and the Ring taken from him, and there at last he died." ~The Return of the King (Appendix A)

 

His fate remained a mystery, however, until Gandalf found him in the abode of the Necromancer.

 

"And after this Meriadoc questioned Gandalf further about Thráin's map and key; and in the course of his reply Gandalf said: 'It was nine years after Thráin had left his people that I found him, and he had then been in the pits of Dol Guldur for five years at least.  I do not know how he endured so long, nor how he had kept these things hidden through all his torments.  I think that the Dark Power had desired nothing from him except the Ring only, and when he had taken that he troubled no further, but just flung the broken prisoner into the pits to rave until he died.'" ~Unfinished Tales (The Quest of Erebor)