The barrel bomb is created when putting a pot of quicklime and ground sulphur on a barrel of naphtha during the Regicide quest. After obtaining the unfused barrel bomb, a strip of cloth needs to be added to activate it. The bomb is then to be shot at King Tyras, killing him in a fiery inferno.
This is one of the heaviest objects in the game, weighing 32 kg. The other objects are the Barrel of coal-tar used to make the bomb in Regicide, and the Obsidian Column used in TokTz-Ket-Dill.