Dragon Age: Ascension
A Blight refers to a period when darkspawn find and corrupt one of the Old Gods, which is transformed into an archdemon and leads the horde to attack the surface world. The world has seen four Blights to date.
A Blight is distinct from the occasional darkspawn raids, in which the darkspawn are few, scattered, and disorganized, plaguing the Deep Roads almost exclusively. These grand highway-tunnels – constructed by the dwarves at the height of their empire but almost entirely controlled by darkspawn ever since the First Blight – extend throughout subterranean Thedas.
When the darkspawn discover one of the Old Gods—ancient draconic creatures slumbering in the depths of the earth—they infect it with their taint, and it rises as an Archdemon. Unified by its will, the darkspawn surge to the surface and spread across the lands, warping and destroying all in their path. Victims not killed are dragged underground to be eaten or turned into Broodmothers, water sources are defiled, crops and animals plagued. The sky fills with black clouds that block out the sun, making it easier for more darkspawn to surface.
The only way to end a Blight is to slay its Archdemon, and the only ones who have ever managed such a feat have been Grey Wardens. For this reason, the order accords legendary respect throughout most nations (though their presence in Ferelden in the decades prior to the events of Origins is weakened by local political struggles and the centuries that have elapsed since the most recent Blight).