Ancalagon (the Black)Ancalagon (the Black) - In a last frantic bid to save himself, Morgoth unleashed a mighty fleet of dragons from Angband during the War of Wrath.  And the greatest among them, indeed the greatest dragon in the history of Middle Earth was Ancalagon the Black.  But in the end, even they could not halt the advance of the Valar.

 

"Then, seeing that his hosts were overthrown and his power dispersed, Morgoth quailed, and he dared not come forth himself.  But he loosed upon his foes the last desperate assault that he had prepared, and out of the pits of Angband there issued the winged dragons, that had not before been seen; and so sudden and ruinous was the onset of that dreadful fleet that the host of the Valar was driven back, for the coming of the dragons was with great thunder, and lightning, and a tempest of fire.

 

But Eärendil came, shining with white flame, and about Vingilot were gathered all the great birds of heaven and Thorondor was their captain, and there was battle in the air all the day and through a dark night of doubt.  Before the rising of the sun Eärendil slew Ancalagon the Black, the mightiest of the dragon-host, and cast him from the sky, and he fell upon the towers of Thangorodrim, and they were broken in his ruin.  The the sun rose, and the host of the Valar prevailed, and well-nigh all the dragons were destroyed..." ~The Silmarillion, chap. 24 (The War of Wrath)

 

When contemplating the destruction of the Ring of Power, Gandalf wondered if dragon-breath might do the trick.

 

"It has been said that dragon-fire could melt and consume the Rings of Power, but there is not now any dragon left on earth in which the old fire is hot enough...not even Ancalagon the Black..." ~The Fellowship of the Ring, Book I, chap. 2 (The Shadow of the Past)