Urthemiel is the main antagonist of Dragon Age: Origins, though his name was not revealed until the events of Dragon Age:Origins - Awakening. He is referred to simply as "the archdemon", suggesting that his identity was only confirmed after the Blight ended.
Urthemiel or the "Dragon of Beauty" was one of the Old Gods, a dragon-like creature worshiped as the god of beauty by the Tevinter Imperium. Urthemiel was awakened by darkspawn and transformed into an archdemon in 9:30 Dragon, shortly before the events in Dragon Age: Origins, which revolve around the Fifth Blight.
Among the ancient Tevinter people, Urthemiel was revered as the most benevolent and most graceful of the Old Gods. His beauty is now a horrid mockery of the being he once was. If not religious canon of the Tevinters, at least some dragon cults believed Urthemiel fathered a daughter, Urzara.
It is revealed that Urthemiel was found not by the main darkspawn horde, but by the mysterious, sentient darkspawn known as the Architect. After a botched attempt to induce Grey Warden blood to Urthemiel, which the Architect believed would disrupt the Old Gods' hold over the darkspawn; Urthemiel became tainted, beginning the Fifth Blight.
Urthemiel is first seen by The Warden in dreams, before actually witnessing the archdemon firsthand at the Dead Trenches, where it commanded the darkspawn horde to move towards the surface.
Urthemiel led the darkspawn horde to attack Denerim, the capital of Ferelden, where it was promptly slain by the last two Grey Wardens remaining in the country. The fifth Blight was the shortest in history, ending the same year it began.