The Hobbit The Desolation of Smaug Deleted Scene - Sauron kills Thrain Full HD - YouTube.

What actually happened to Thráin?

Many Dwarves believed that Thráin was driven mad by grief and had been among the fallen, but his son Thorin had searched among the dead bodies of Moria and knew that Thráin had survived the battle. However, it was revealed that he was captured by the Necromancer and became a prisoner in Dol Guldur.

Why did Sauron capture Thráin?

ANSWER: J.R.R. Tolkien never explained, so far as I know, why Sauron decided to take back the Rings of Power he had given to the Dwarves. We only know that he seized Thrain and took his Ring from him, “the last of the Seven”, according to Thrain. ... He apparently wanted to use the Rings to control those races.