Black Arrow

The Black Arrow was an heirloom of the house of Girion, Lord of Dale, passed down to Bard, his descendant.

History
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).

The Hobbit film trilogy
In The Hobbit: The Desolation of Smaug, the Black Arrow is not a single regular-sized arrow, but a series of large metal arrows designed to be fired by a windlass crossbow at 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 attempted to kill Smaug, although his last shot damaged one of Smaug's scales. When Thorin and Company accidentally released Smaug on Lake-Town, Bard attempted to prepare the windlass to attack Smaug, but he was arrested by the corrupt master of Lake-Town and his son was forced to hide the Black Arrow in a boat for later.