Bolg is perhaps the mightiest of all the Orcs (or Goblins) of the northern world in the time of the story. He is the leader of the Orcs of Mount Gundabad, their chief fortress in the Misty Mountains. His father was Azog, who got killed at the gate of Moria by Dain in the war of the Dwarves and Orcs. Bolg ruled Moria for some 150 years.
He became very angry over the killing of the Orc captain of the High Pass by the Company and therefore attacks the Dwarves of the Lonely Mountain, when he hears the news from there – something that proved not very good for him. But by that he ended the feud between Dwarves, Elves and Men there.
He led an army of Orcs in the Battle of Five Armies. He was killed by Beorn, where it says: “Swiftly he returned, and his wrath was redoubled, so that nothing could withstand him, and no weapon seemed to bite upon him. He scattered the bodyguard, and pulled down Bolg himself and crushed him.”