Archdemons are the Old Gods manifested in the form of powerful and terrifying dragons, and tainted by darkspawn. They possess an intelligence far beyond the average dragon and are purely evil creatures.
Normally, the darkspawn are disorganized and fight with each other, rarely appearing on the surface except in raids and small invasions. However, a true Blight begins when the darkspawn find a leader who unites them into a great horde and unleashes 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.
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.
The first appearance of an archdemon in the game occurs when the player character drinks the tainted blood of the darkspawn during the Joining. However, when the Player Character travels to the Dead Trenches, they will see the Archdemon in the flesh during a cutscene, but will be unable to interact with it.