Who was the leader of the 500 dwarves who go to help Thorin in The Hobbit?
The answer to this question can be found in Chapter 17, which is when the dwarves under Thorin's rather crazed leadership are trying to keep all the gold from the mountain that they feel belongs to them from Bard and the Elvenking. Goldlust has fallen upon Thorin and made him obsessed with keeping the gold and full of greed, and so Bilbo has carried out one of the many truly heroic actions he performs in the novel: he steals the Arkenstone from Thorin, the jewel that he knows is most important to him above all else, in order to prevent bloodshed, as he knows that Thorin will have to negotiate to get back his stone. However, the situation is made more messy by the arrival of Dain with five hundred dwarves to support Thorin:
The morning was still early when a cry was heard in the
camp. Runners came in to report that a host of dwarves had appeared round the eastern spur of the Mountain and was now hastening to Dale. Dain had come. He had hurried on through the night, and so had come upon them sooner than they had expected. Each one of his folk was clad in a hauberk of steel mail that hung to
his knees, and his legs were covered with hose of a fine and flexible metal mesh, the secret of whose making was possessed by Dain’s people.
It is important to remember that at this stage in the novel the arrival of Dain's troops represents a real threat to the achievement of peace. Thorin is less likely to come to terms when he has the support of so many strong warrior dwarves to fight with him against the humans and elves, and therefore the arrival of Dain represents an increase in tension as we as the reader wonder what is going to happen to relieve the situation.