Ancalagon the Black was a dragon in J.R.R. Tolkien's fantasy universe of Middle-earth.
The Dark Lord Morgoth bred Ancalagon during the First Age to be the mightiest of all the winged dragons. During the War of Wrath that pitted Morgoth's armies against the host of the Valar, Morgoth sent Ancalagon forth from the fortress of Angband to destroy the Dark Lord's enemies. So powerful was the dragon that even the Valar host was driven back.
But Eärendil, a Half-elven man descended from the princes of the Eldar and Edain, descended from the sky on his ship Vingilot and duelled with Ancalagon. They fought for an entire day until Eärendil achieved victory and slew the dragon, whose ruined corpse crashed down upon the triple-peaked towers of Thangorodrim, destroying them. With his mightest defender slain, Morgoth was utterly defeated soon afterward.