Kilgharrah is the dragon that Merlin meets and goes to seek advice in the TV series "The Adventures of Merlin". At the end of episode 12 in season 2, Merlin releases Kilgharrah from his chains, after being held prisoner by King Uther for many years. In episode 13, Kilgharrah begins to terrorize and attack Camalot in a fit of revenge; in the end, Merlin defeats Kilgharrah.
Merlin speaks to Kilgharrah in the language of the Old Religion, which is a magical language understood by dragons and sorcerers. This language is ancient and carries powerful mystical properties.
KilgharrahQuote :Balinor: What do you want from me?Merlin: Are you Balinor? The Great Dragon is attacking Camelot.Balinor: His name is Kilgharrah.
In BBC's Merlin season 2 finale. we find out that his name is Kilgharrah. He is known by that name to the dragon lords, and presumably to everyone else too. Balinor was the first to reveal his name.