Archdemons

Role in the Blight

Normally, the Darkspawn are organized as a simple hive-mind concerned only with expanding the horde and finding an Old God. They emit a sound that sounds "beautiful" to the Darkspawn, compelling them to not rest until they find its source. They appear on the surface except in raids and small invasions.  
 
 
However, a true Blight begins when the Darkspawn find a leader who is able to unite a great horde and unleash them onto the surface in a wave of darkness and destruction. This leader is an Archdemon and a Blight will not end until the Archdemon leading it is destroyed.   

Old Gods

To date, five out of the seven Old Gods have been corrupted by the Darkspawn and subsequently defeated by the Grey Wardens. 
  
Dumat, the Dragon of Silence and the first Archdemon 
Dumat was first among the Old Gods, and ultimately the first Archdemon. He was known as the "Dragon of Silence" prior to being imprisoned underground by the Maker for usurping his worshipers. Some time after their initial corruption, the Darkspawn burrowed into Dumat's underground prison, where the Old God was corrupted by the Blight, becoming the first Archdemon. Dumat was slain at the Battle of Silent Fields. (-223 Ancient) 
 
Zazikel, the Dragon of Chaos, and Archdemon of the Second Blight 
Zazikel was the Dragon of Chaos and was awakened in 1:5 Divine by the Darkspawn. This led the Second Blight and fall of Nordbotten in the Anderfels. After many battles, an Orlesian army under command of the emperor Kordillus Drakon defeated Zazikel at Starkhaven, thus ending the second Blight. 
 
Toth, the Dragon of Fire, and Archdemon of the Third Blight 
The Dragon of Fire was was awakened by the Darkspawn in 3:10 Towers, incurring the Third Blight. 15 years of conflict ensued, before the Grey Wardens, the Orlesians and the Tevinter forces united in Hunter Fell and destroyed Toth, ending the third Blight.  

Andoral, the Dragon of Chains, and Archdemon of the Fourth Blight 
The Dragon of Chains was awakened in 5:20 Exalted by the Darkspawn and was the Archdemon of the Fourth Blight. An army of Grey Wardens led by the elven hero Garahel, freed Antiva of the Darkspawn in the battle of Ayesleigh. Garahel defeated Andoral, ending the fourth Blight.  
 
Urthemiel, the Dragon of Beauty, the Archdemon of the Fifth Blight, in Ferelden 
Once the Tevinter Dragon of Beauty, Urthemiel was worshiped by musicians, artists, and poets in ancient times. The Feast of Urthemiel was the largest celebration of the year, an event that lasted a full twelve days. Plays and entire symphonies were written in his honor. Now, he is a maddened husk of his former self, filled with nothing but a desire to destroy all life.   

Razikale, the Dragon of Mystery  

Lusacan, the Dragon of Night 
He is the Tevinter Dragon of Night and is worshipped by the Tevinter Imperium. Lusacan is one of the slumbering old gods and is yet to be corrupted by the darkspawn.

Appearance in Dragon Age: Origins

The first appearance of an archdemon in the game occurs when The Warden drinks the tainted blood of the Darkspawn during the Joining. However, when the Warden travels to the Dead Trenches, an Archdemon will be seen in the flesh. The Archdemon in the game was a Tevinter god of beauty named Urthemiel.

The Warden discovers on the eve of the Battle of Denerim that the Archdemon must be killed by a Grey Warden. If killed by anyone else, as Riordan explains, it will move into the body of the closest Darkspawn (who are soulless). This makes it almost immortal, unless one were to endeavour to eradicate every Darkspawn. However, if a Grey Warden is the one to kill it, the Archdemon will seek out the body of the Grey Warden. The result of two souls attempting to occupy the same body causes both to be destroyed. As the eldest surviving Grey Warden, Riordan volunteers to be the one to slay the Archdemon.

The capital of Denerim is the site of a major Darkspawn offensive and this is where The Warden will fight the Archdemon, with the help of the allies gained by using the Grey Warden treaties. At first there is no real sight of the Archdemon until it is seen flying over the city followed by a charge of human soldiers.

The next sighting of the Archdemon is at the Alienage, as the Warden and allies take their leave of the area after saving the Alienage (or possibly leaving it to a grisly fate) and disposing of the Hurlock General there, the Archdemon will make an appearance - flying overhead and destroying the bridge making it impossible for the group to turn back.

Along with the remnants of whatever troops Riordan had led into the area dealing with Darkspawn, Riordan himself will be atop a tower where he ends the life of a Darkspawn before the Archdemon takes notice of his presence. The Archdemon swoops past the tower and Riordan jumps onto the Archdemon's back, keeping himself on board as it flies through the air by stabbing his sword into its back. The Archdemon struggles to be rid of Riordan and eventually tosses him off, but in the throw Riordan pierces the Archdemon's wing and slices through it as he falls off to his death. The Archdemon however suffers from the blow and crashes into Fort Drakon where it awaits the Warden while it slaughters whatever soldiers are stationed there. 
 

Codex Entry

"No one knows what it is that drives the darkspawn in their relentless search for the sleeping Old Gods. Perhaps it is instinct, as moths will fly into torch flames. Perhaps there is some remnant of desire for vengeance upon the ones who goaded the magisters to assault heaven. Whatever the reason, when darkspawn find one of these ancient dragons, it is immediately afflicted by the taint. It awakens twisted and corrupted, and leads the darkspawn in a full-scale invasion of the land: a Blight."     

Trivia

• Since the dwarves managed to fend off the Darkspawn invasion of the Deep Roads after the First Blight, no Archdemon has led a Blight against them.
• Dumat may be a reference to "Duma", who, in Jewish folklore, was known as the "Angel of Silence."
• The name "Old God" could possibly be an H.P. Lovecraft reference. In many of his writings he refers to ancient, often malevolent, cosmic deities of great physical and spiritual power capable of driving men to madness. These include the infamous Cthulhu and many others, some of which have an appearence similar to a twisted and unnaturally corrupted dragon much like the Archdemon.
• Toth, the Archdemon responsible for the Third Blight, may be a reference to either Azathoth or Yog-Sothoth, two Outer Gods of Lovecraft's Cthulhu Mythos.
• The Chant claims that Archdemons and the Blight are the Maker's punishment for the world's idolatry of the false Old Gods.