A Blight refers to a period when darkspawn find and corrupt one of the Tevinter Old Gods, ancient draconic creatures slumbering in the depths of the earth, which is transformed into an Archdemon by the taint and leads the horde to attack the surface world.
A Blight is distinct from the occasional darkspawn raids, in which the darkspawn are few, scattered, and disorganized on the surface, plaguing the Deep Roads almost exclusively. These grand highway-tunnels were constructed by the dwarves at the height of their empire, but has been almost entirely controlled by darkspawn since the First Blight—extending throughout subterranean Thedas. A particularly strong or clever alpha can lead darkspawn war bands to cause great devastation on the surface even when an Archdemon is not active in the world, and can even corrupt the lands those war bands attack with the darkspawn corruption at a lesser scale, but if the leader is defeated, the war bands will disperse and the land will slowly heal.
Blights advance in a progression that the Grey Wardens understand relatively well. However, Warden scholars remain uncertain about the precise “rules” under which Blights operate. Does an Archdemon’s presence cause and spread the Blight, or is it the hordes of darkspawn that invariably travel with them? Does it take all Archdemons a similar amount of time to reach the surface? What would happen if an Archdemon were left unchecked forever? Even for the gaps in their knowledge, the Grey Wardens use what they do know to carry out their vigilant work. The following is what is known of the common progression of a Blight.
Origin of Blights
When the Evanuris were still primeval spirits who wanted to become corporeal, they used raw lyrium to craft physical bodies for themselves, thus becoming the first Elves. In the process, however, they angered the Titans and entered into a war with them. When the elves were losing the war, Mythal had Solas craft a Lyrium Dagger which Fen’Harel used to sever the Titans’ connection to the Fade, rendering them Tranquil. The Titans’ dreams, now severed from their hosts and locked away inside the Fade, mutated into the Taint and Blight out of anger, fear, and confusion. Solas banished these corrupted dreams which would become the Blight to a prison which would be known to contemporaries as the Black City.
The Evanuris weaponized the Blight in their wars of supremacy, especially during Solas’ rebellion against them. For unleashing the Blight onto the world, Solas imprisoned the Evanuris in the Fade and used their lives to sustain the Veil. Yet Solas did not anticipate that the Elven pantheon that he had imprisoned would speak through the Old Gods and manipulate the Magisters Sidereal into breaching the Black City and releasing a portion of the Blight onto Thedas.