"There's a shit-ton that can be said about Blights, but I'll try to condense it as best I can. Blights are when darkspawn -tainted humanoid beings- find a sleeping Old God and taint it. Said Old God becomes an Archdemon and the driving force behind their actions, leading them to the surface. They actively kill or corrupt anything in their path, but their very presence also taints the land, water and air around them. Everything suffers, and will continue to suffer until the Archdemon is slain. That's why the Grey Warden order exists, to fight darkspawn and destroy the Archdemon each time a blight surfaces.

So far, Thedas has had five. The most recent one was actually the least destructive, in that it didn't go beyond Ferelden's borders. Most end up devastating the entire continent."