- "I rule over your land, Free Peoples, and my overlord is the One, the one who you dare call "Enemy". Now you face me for I am the Dark Lord's wrath!"
- ―Thanos
"Covered in black scales like his sire Ancalagon the Black, slender as Smaug the Tyrannical, greedy as Scatha the Greatets of the Long-worms and fiery as Glaurung the Father of Dragons is he Thanos, he who sits on the Dark Lord's throne"
Thanos is among the meanest Dragons to have ever existed on Middle-earth. Since his birth, Thanos displayed sadistic tendencies as the first victims were his smaller siblings, whom he devoured alongside his own mother. This act of savagery greatly impressed the Dark Lord Sauron, who kept his existence a secret. His father was Ancalagon the Black, greatest of the fire-breathing dragons and the One filled him with a desire to be even more powerful than his sire.
After his Master's defeat in Dagor Dagorath, Thanos began laying waste to Middle-earth and strived to conquer the land and rule it until Sauron's return (despite the fact that the Dark Lord has been rendered a spirit of malice unable to manifest itself or even return).
He rallied hordes of Goblins, Trolls, Giants and lesser Dragons under his banner and began his campaign against the Free Peoples in the Fourteenth Age. He came to Carn Dum, once King Mûrazôr's domain, and demanded that the fire-drake Urgost serve him. Unwilling to give up without a fight, the latter declined and the two dragons fought until Thanos bit his neck down and forced him to kneel. The older creature tried to strike back, but the son of Ancalagon clawed him and before putting his heavy arm on it's head, crushing it. Having defeated his rival, the Dragon declared himself Lord of Carn Dum, before devouring Urgost's corpse whole.
He suffers a psycyhological mentality believing the only thing worth living in middle earth his the absence of life.
In the Sixteenth Age, the Vala Tulkas came to Middle-earth and used a spear blessed by Eru Iluavatar to slay the beast, ending it's brief reign of terror.