Lonely Mountain
Smaug of Erebor, the Lonely Mountain, was killed by Bard in Dale, as told in "The Hobbit". The Arkenstone was buried right in the pile he slept on, but Smaug never even noticed it. Smaug had only a single weakness: there was a hole in his jewel-encrusted underbelly on his left breast area. Bilbo Baggins discovered this, which led to Smaug's death above Esgaroth.