The Black Arrow was Bard the Bowman's last arrow remaining when Smaug attacked Laketown and, directed by a thrush who had overheard a conversation between Bilbo Baggins and Smaug, he shot it with remarkable speed into a weak spot in the left part of the chest of the dragon's natural armour of scales, killing him, and thus freeing Lonely Mountain (Erebor), Dale, and Laketown.
The Black Arrow was forged by Thrór the Dwarf, who was King under the Mountain (Lonely Mountain or Erebor), according to The Hobbit. It is not known if it had any magical properties, but Bard said that he had successfully retrieved it every time he used it (like Beleg's arrow Dailir).
Portrayal in adaptationsEdit
The Hobbit film trilogyEdit
In The Hobbit: The Desolation of Smaug, the Black Arrow is not a single regular-sized arrow, but a series of large metal quarrels, something between a spear and a grappling hook, designed to be fired by a windlance crossbow at attacking dragons. During Smaug's original assault on Erebor that culminated in him taking control of the Lonely Mountain, all but one of the Black Arrows were used up as Bard's ancestor Girion, attempted to kill Smaug in defense of Dale. His last shot damaged one of Smaug's scales, right before the dragon killed the Lord of Dale. When Thorin and Company accidentally released Smaug on Lake-Town, Bard attempted to prepare the windlance to attack Smaug with a Black Arrow, but he was arrested by the corrupt Master of Laketown and his son was forced to hide the Black Arrow in a boat for later.
In the Battle of the Five Armies, Bard the Bowman shot Smaug from a churchtower with a black arrow, using his son as a stand and rope from the church bell as his bow. Bard hit the weak spot that the Lord of Dale had made years before, in the first attack of Dale by Smaug. Smaug died in the sky, and dropped on the corrupt master of Laketown, killing him and sinking his boat.