Though Glaurung was named Father of Dragons, Ancalagon also known as Ancalagon the Black was the most powerful and infamous of all the dragons bred by Morgoth during the first First Age. He is the largest Dragon to have ever lived and the greatest of all winged Dragons, as well as His fire was hotter than most other dragons.
Morgoth bred Ancalagon sometime in the late First Age of the Sun. Ancalagon was intended to be the Angband's last defence of an invasion, he would crush the enemy and obliterate them. Morgoth made Ancalagon far more powerful than even Glaurung, his ancestor. Unlike other Dragons, Ancalagon could fly. He was the first of his kind to do so.
War of Wrath
He burst out of Tharangorodrim volcano in the War of Wrath and terrified the Host of the Valar, making them retreat and consider ending the war, because of the immense casualties caused by Ancalagon's devastating fire. When it looked like no hope, Thorondor the Eagle and all other Eagles came, and they battled with Ancalagon and the other Dragons.
Soon, almost all Dragons were defeated, and Thorondor fought Ancalagon throughout the night. As dawn broke, Thorondor got the upper hand, and killed Ancalagon, hurling him out of the sky, and the Sun came up upon his death, as Ancalagon smashed onto Tharangorodrim, smashing it to ruins. Ancalagon's death ensured the victory of the Host of the Valar.
Gandalf, in the Third Age of the Sun, said that Ancalagon was probably the only Dragon, and the only being, capable of destroying a Ring of Power, even the One Ring. Being a Maiar, Gandalf would have probably been in the War of Wrath, and could've seen Ancalagon personally.
- Ancalagon is a Sindarin word usually translated as 'Rushing Jaws'.