"His breath is unllke any dragons for it has been responisble for more deaths than every Dark Lord past or since."
Thanos is among the eviliest Dragons to have ever existed on Middle Earth. Since his conception Thanos was evil incarnate. the first victems were his own parents where he used his unique breath weapon against them. He lay waste to middle earth constantly looking for his most precious element, Obsideon.
By the fourth age he single handely wiped out all the men from the Cara Dum. He suffers a psycyhological mentality believing the only thing worth living in middle earth his the absence of life.
He was slain in the 16th age by a more modern weapon that was designed especially for him.